Hmm I can see your worries but I think its too late for you to get another 'long run' in now.

I think it depends what you want from your marathon? A friend of mine only went up to 14 miles in training and finished in just over 6 hours.

I'd say you've covered enough distance, you don't want to do too much.

I did a 15, 17, 18 and 2 20 milers which left me injured and running the majority of my marathon in pain!

It really depends on what your goal is. If you want to go out there and "run:" 26.2 miles then I would say no, you aren't ready. If you want to complete the marathon regardless of your time then you will probably be fine. I suggest you find the pacer for the slowest group and make friends. Start off slow and walk for all of your water breaks, even early on.

Whilst there is a lot of evidence to sugget that you don't need to complete a long "long run" the thing to consider for first timers is the psychological aspect of running / completing a marathon. That's where the real benefit of the 20 milers come in, knowing you only have a 10k run to go!

Slightly off topic for your situation but pertinent to a marathon. MFP encourages folks to drink, drink, drink and it's clear that there is limited value to consuming a lot of water while dieting (it doesn't actually help you lose weight but it does help loosen your bowels and the bloating helps cut back on the amount of food people eat).

When it comes to running, though, drink, drink, drink can kill people. And it sounds like you are in the cohort where most of the problems occur.

The issue is hyponatremia and the "typical" victim is a someone who runs a marathon slowly or does a run & walk marathon and makes sure to "stay hydrated" by drinking a lot or even carrying a water bottle with them. In that the runner is not perspiring heavily yet they are consuming lots of water, what they are unknowingly doing is diluting the electrolytes in their blood. As they drink more, the heart beat becomes irregular and, if things get diluted enough, over they go.

The situation is worsened by EMT's who may not be trained to recognize hyponatremia and, instead, they see a heat injury - shallow breathing, low pulse, etc. Standard response is an IV and with that, things go from bad to worse.

For the background on this, check out:

http://www.outsideonline.com/blog/outdoor-adventure/tim-noakes-on-the-serious-problem-of-overhydration-in-endurance-sports.html

Dr. Noakes is a legend in the running world (he invented the energy gel and is also the author of "The Lore of Running") and has been instrumental in getting sporting bodies to change their recommendation from recommending "pre-drinking" and "forced hydration" to their current position of "drink to thirst".

<another_PSA_from_the_MFP_water_curmudgeon> :-)

Sounds like you are tapering. relax, and go into the race knowing that you did your best during training. That 20 miler is still a 20 miler, and you did awesome regardless. There are more components, but think simple. During the taper, there isn't much you can do to increase your performance, so even thinking about doing another long run is out, BUT there is a lot you can do to screw it up. just relax. Get to the starting line injury free and well rested. You will do fine. Trust your training.

im training for my first marathon in october but for my 1/2s i still take the occassional walk break(finished my last in 2:25, which i don't think is half bad for walk breaks) My goal for the marathon is around 5 hours and i will still be taking my walk breaks. If your plan is to finish i see know problems.