Tapering is extremely necessary, especially for the first marathon. The normal suggested tapering period for a full marathon is 2-3 weeks. It gives your body and mind plenty of time to recover. Studies actually prove that proper tapering increases your finish time and overall experience of the race.

Jumping from 28km to 80km a week in 1 months time (with 3 weeks to taper) is pretty drastic. But don't worry, you are not behind schedule. I'd suggest working up to a long run of 20 miles about 3 weeks before. This gives your body plenty of time to heal and your mind plenty of time to recover because 20 milers are probably more mentally exhausting than anything. Once you run 20 miles you can tackle 26.2.

Good luck!

I would recommend that you start using a training plan. Based on the questions you have been asking over the past several weeks it is obvious that you are feeling anxious about being prepared for your marathon. A training plan will take care of a lot of the guesswork that you are currently doing and allow you to concentrate on training and recovery, instead worrying about how much distance, how often, and how fast.

Personally I have always used Hal Higdon's plans and I have no complaints. Since this is your first marathon use a novice plan and count back from the end to figure where you should be in the plan.

I am currently training for a marathon in April using an intermediate plan and my biggest mile weeks have a total of 43 miles (<70km), so hitting 80 or 100 km is not necessary. Also you say that you are adding more distance each week, but you should be building up your distance and then every 3 to 4 weeks have a step back week where you drop your distances for your long run and intermediate long run. The purpose of a step back week is to allow your body to recover from the pounding and fatigue that you have been accumulating.

Most training plans will have you maxing out with a long run of 20 miles (32 km), because this is the tipping point where the training benefit still outweighs the damage and fatigue created by running that distance.

The purpose of tapering as you approach race day is so that your body can recover, eliminate the built up fatigue, and be fresh for race day. If your body is not ready to cover the distance two weeks out, then trying to cram in extra miles during those final two weeks is not going to get you to the start line any more prepared. Being rested and recovered on race day will help you stay under the cut off time.

So my advice is:
1) Find a plan and follow it. Just remember it is a guide and if you need to shift which day you do the long run or which day(s) are your rest day then go ahead and make the adjustments.
2) Run smarter not farther or faster.
a) Running extra distance is just junk miles and increases your chance of developing an injury and not being in top condition at the start line.
b) Follow the 80/20 rule. 80% of your training runs should be at an low intensity/easy pace and only 20% should be at moderate or high intensity (http://www.runnersworld.com/rt-web-exclusive/train-at-the-right-intensity-ratio). In training plans this might be stated as running at 60-90 seconds/mi slower than your planned race pace. I think that works out to 35-56 seconds/km slower.
3) Get plenty of rest. If your plan says it is a rest day then rest. You can do light activities like walking, yoga, and stretching. However don't use it as a day to squeeze in a spin sessions or an extra run.
4) Use some of your long runs as a race day simulation. Get up when you will for your race, have breakfast, go for your run. Figuring out what you can stand to eat before a long run can make or break your race day experience.
5) On race day start toward the back and start slow. If you start in the front half of the pack you will be sucked along and end up running too fast too soon. This will only set you up to have a bad second half. Psychologically it is better to start toward the back and pass other people during the race, than to start toward the front and watch the pack stream by you.

If you want to add more miles make sure they have a purpose. That is why everyone is giving your the advise to get a training plan and stick to it. Those plans typically have the runs set up so each one has a purpose. Adding 20k (12 miles) to where you are at at this point in time would be difficult but not impossible. A few ways to add some miles with purpose would be something similar to these runs if you currently do not do them. If you already do them then ignore the following.

If you do not do a recovery run the day following your long run, one that is 4-6 miles at a super easy pace, you could easily add there. Make sure it is at least 75-90 seconds per mile, or more slower than your goal pace. Recovery runs are a good way to get some blood flowing to your stiff legs before continuing with some harder training in the form of tempo's or intervals the following day.

If you do not do an easy pace run the day before your long run you could add some miles there as well. Those miles would help put some fatigue in the legs on your long run and help simulate those last few miles of a marathon. All the plans I've ever done have you doing half the distance of the long run the day prior. If you don't do that now it wouldn't be wise to add 10 miles the day before your 20, but doing a 5-6 mile run would still have a purpose. Just make sure it is at an easy pace.

Adding in short 2-3 miles run would be what one poster described as junk miles so avoid the temptation to just go for a quick jog just to add a couple miles to you total. Adding miles is not always a bad thing, just make sure they have a purpose in your training. I personally don't think that a peak of 60-62 miles is a bad thing, just make sure that you do it gradually and make those miles easy runs.

There's not really any reason to go above 20 miles in training, nor above 80km for a weekly total. You're doing a lot all at once: you're putting more stress on your body by running the longer long run as well as adding on more and more mileage to get to that 80 km each week. At this point in your training, run 20 miles/32km at least once, and then start tapering the first weekend in April.

Honestly, I think your focus should be getting to the starting line healthy considering what you're putting your body through in training up so quickly from HM to full marathon distance. You're trying to do in two months what most people would build up to over 4-6 months, or even longer. My best advice to you would be to make sure you get in an 18 miler, a 20 miler, and a solid taper, then get to the starting line and finishing line in one piece.

Edit to add: Wait are you running 28km TOTAL for the week or is that your long run currently? If that's how far you are running over the whole week, that does not bode well. You are not going to be able to almost triple your mileage in the next 6 weeks without incurring injury. If that is your long run and you are building up to 80km from maybe 60, then you could maybe do it. Carefully.

Good luck! My opinions below...

Your long run right now is appropriate for a full marathon distance. As others have mentioned a 32km once can help, but do an honest check with yourself that you are ready to try that first. You would be better served having done long runs of 28k and making it to race day healthy than feeling you must reach 32k and hurt yourself. Some believe the 32k is not even necessary, but I believe it provides some mental preparation for race day. It's clearly not an absolute necessity, but a nice to have in your case.

Your weekly mileage is a bit low, but not low enough to panic. Considering you only have about 3 more weeks of training before you start your taper you cant build up much higher, but might finish these last 2-3 weeks before the taper with more than 60k, perhaps 65 or even 70 at the very most. As much as you can do feeling quite safe about it. Bring healthy is most important this close to the start.

Don't change your training now by taking up a training plan this close to the race, there is not enough time to benefit from it, but plenty of opportunities to try something you're not physically ready for that the plan suggests you do. That could cause injury and all of your efforts could be lost.

You mentioned you have only been running for 6 months, yet have come so far. Clearly you were in great shape starting out. I think your maun focus should be avoiding injury because your tendons and bones have only been adapting to 6 months of extended pounding from running. So don't worry about speed, just run comfortably and get the kilometers you can ahead of the taper. Run the race similarly because that will be the most demanding effort you have asked if your body, so no sense pushing or straining at any point of it unnecessarily. Taper appropriately and eat healthy, load up on carbs ahead appropriately (don't assume just a plate of pasta the night before is proper marathon carb loading).

Most of all...have fun. Even if you feel miserable and think you want to die, put on a smile and try to laugh, even if it seems impossible at least try to push yourself to finish happy. Because when you finish you WILL be very happy.

Please update us with how things go!

Yes. Glad to hear that 28km is your long run and not your weekly total. I suspected as much, but you never know. You would be surprised at the posts that show up on here! Anyway, I agree that it sounds like you are in pretty good shape and have prepared well. Now you just have to trust in your training. As simple and cliche as that sounds, it's probably the number one piece of advice for anyone going into a marathon, be it their first or their 20th. If you trained sufficiently, you will do fine.

You know, it is kind of interesting to see how this thread with 1.5 months of training before a full varies from another with 6 weeks left before a half. The main difference is in the current training condition of the runners. Doesn't help the OP and hope this is not considered a hijack, but I thought it was interesting reading my advice and others here again after some thoughts yesterday in the other thread. I know it is apples and oranges in a number of respects, but this one is more cautious and the other one much more bold.

@WhatMeRunning @lporter229 - I didn't offer advice on either thread, because a) others had already covered the most important points, and b) the situations and conditioning of the posters are enough different than mine that I'm not sure how relevant my experience is to them.

THBS, there are two considerations that jump out at me. As @lporter229 observes, a full marathon and a half are different beasts. The half is much more attainable in terms of time required to prepare and how well it forgives a runner for being light on the training. The other thing is, the runner asking about a half has already done one, and the runner asking about the marathon is doing her first one. There is a huge difference between preparing for the first half or full marathon and preparing for a half or full marathon with the experience of having already completed one. Particularly for the full marathon, conservatism is indicated for the first time. No amount of reading and conversation with marathon runners can totally prepare you for how miles 22 to 26 of a marathon will feel. And there's always that nagging doubt about the ability to actually run 26.2 miles, until you do it for the first time.

The first vs. second thing struck me yesterday evening. Before my first marathon, I talked to the man who would later be my coach. When he heard it would be my first, he said things like, "The first 10K is a warmup," and, "Don't pass the pacer." Now I'm following his training plan for my second marathon, which happens to be Boston. Now he's saying things like, "Run the first 10K at 10 seconds per mile faster than your average marathon pace for goal time, then slow down to MP and try to hold on." The coaching for the first was coaching to finish; the coaching for the second is coaching to finish well and hit a goal time on a challenging course with the easy part early and the hard part starting at mile 16.