Marathon Training and Dieting

Personally I don't specifically eat more the day before a long run, I want it afterwards.

If you eat normally your body will be fuelled for the run. I can happily run at least 10 miles powered by breakfast, it's miles after that I would start to have the occasional bit to eat.

Remember to log your exercise and eat back those calories. As training progresses I tend to worry less about the size of the deficit and more about the nutritional value of what I'm eating. So maintenance is fine if I feel like I need it, but it has to be proper food!

I find that although my weight doesn't really change when I'm running a lot my shape does.

Its worth keeping notes about your training and how you feel, it helps to spot patterns about what works best for you in terms of timing, and exactly what food you get on best with - for before/during and after the run.

Better to try to lose the weight now than when closer to the event, and doing longer runs that cause more stress on body, which diet will do also.

Confirm you using MFP correctly to try to get a reasonable deficit off your Total Daily burn.
Bigger extreme deficit won't be useful during training, frankly ever.

With 42 lbs to lose - 1.75 weekly could be reasonable - 750 cal deficit, until you are at 30 lbs left to healthy weight.
Then switch to 500 cal deficit until 15 left.
Then 250 to the end, or other alternate method like every other week diet still at 500 deficit - which I would suggest actually.
At the times you reduce the deficit, I'd suggest a break week also, eat at maintenance. Reset hormones, aid recovery, ect.

You better be eating more when you do more, and conversely less when you do less.
Life lesson on weight management right there, MFP is trying to teach by having you log exercise when done - and eat more on those days.
Deficit is off daily burn, and obviously exercise adds to that, 10 mile run big time.
Talk about terrible recovery from a harder workout taking a bigger deficit then. Of course if next day is rest day can eat more then and balance between the days - that works too.
But ignoring it is recipe for injury.

To improve accuracy on the running burns - use this with NET option, log it on MFP, and eat it all back.
https://exrx.net/Calculators/WalkRunMETs

Is your MFP Activity level honest?
That could create bigger deficit you don't need.
Many chose Sedentary when they have kids, home responsibilities, and not Sedentary on average for a week outside of exercise.

My concern is the running will be a stress on the body, the training plan is there to lessen it to manageable level.
But so easy with diet to start gaining stress induced water weight, and if that is happening the body will usually have already done other adapting that is not good when doing that training.

Since as a woman it takes a month to have good figures for weight loss to discern something, and this training is ramping up quicker than monthly - you could easily cause some issues between weigh-ins before you could adjust.
My thought is use the best figures possible, and still attempt to adjust as you see results on the body.
But stress water weight will foul that attempt.

To add for your last question about fueling before the race.
No diet that week of course, up the carbs. Night before with traditional carbo-loading is not really that, and not as useful as just being high all week long to confirm topped off.

For no refueling during the run - that's all about training, and not going out faster than you should burning off the limited muscle stored carbs.
That pull of faster pace with other runners around you always gets people in to trouble - not warmed up for one thing so higher carb% being used anyway, and then faster pace at that worst time really using carbs up.
Get in to correct pace group if they do that, and hold to it, perhaps even slower group and speed up after 30 min if needed.

3 stone by October is a massively aggressive goal under any circumstances - never mind while marathon training.

The reason it’s difficult to lose weight while training for a marathon is that your body needs fuel to complete and properly recover from your runs. Early in the training, you will probably be able to manage a modest deficit (maybe a pound a week-certainly not the 3 pounds a week that is your goal).

But as the training progresses, all of your runs will be longer, the longest of your runs will be very taxing, you’ll find yourself with some training fatigue (kind of dreading or resenting running because you’re just kind of over it), along with added personal stress since you will be spending a lot of time running (time that you’re used to spending doing housework/errands/being with friends/family/partner). You don’t just go bang out a 20 mile run once a week then have nothing to worry about until you go bang out another 20 mile run the following week. It’s progressive and cumulative (which it is supposed to be). As the runs get longer, they will take more to recover from in order to even do your shorter runs during the week.

Running a deficit is just not compatible with strong recovery - and that is really very important when you’re stressing your body with a lot of running. Recovery is important to be able to maintain your training plan but also help keep you from getting injured.

I find that I’m able to run a bit of a deficit (less than a pound a week) until I’m at the point where my long runs are around 16 miles. That’s factoring in everything that goes into training during those weeks (longer mid week runs, less time with family, less time for general life maintenance, plus recovering from longer long runs). You might find that you can handle a deficit for a bit longer, you may find you can’t handle one that long. That’s going to be an individual thing.

There’s also a very common phenomenon where people are absolutely ravenously hungry after long runs. When I get over about 14-16 miles for a long run, I am absolutely insatiable after - routinely consuming 4k+ calories on long run day and still being hungry. Not everyone has this issue, but it’s another factor that makes weight GAIN during marathon training a pretty common thing.

On the issue of mid-run fueling…you’re going to need it during your marathon. If you don’t have a tried and true fueling system that you’ve used successfully in the past, then start experimenting and practicing now. Most people need fuel of some sort on runs over 2 hours. You may find gels to work fine. You may find gels cause all kinds of stomach issues and you need to look into other options. Start trying them now so you’ll know how they affect you and you don’t get surprised by a massive stomach ache when you’re out for your first and only 20 miler before the big race. Will that sacrifice your ability to be some super athlete who runs on fat only and never needs a single carb or to have more than a drop of water ever-no matter how hot or how long they run? Sure. But you’re going from 4-6 mile runs to marathon in 3 months while also wanting to lose 45 pounds. Be doing that training. Train to be able to finish 26 miles. That’s already a big enough thing to tackle.

Good luck and have fun! And for each of my marathons-the training has been way more grueling than the actual race (something to think about when the training gets a little rough).