AFAIK you should not run marathons even at 250lbs, due to high impact and its effects on your lower joints. It's better to just walk and do some low impact exercises in order to drop your weight to around 200lbs before trying to run. At least that's what the doctor (an orthopedist with supposedly lots of experience in sports medicine) told me when I was around 250lbs. But I'm 5'9 tall, so if you happen to be much taller than me then I guess all bets are off.I got really fat hitting my peak last year at 350. I was sick all the time had acid reflux , heart burn and after eating i'd be in the bathroom all the time. So I've started and failed the 5k training multiple times but while doing it from nov till june I droped 50lbs and I've been kinda stuck at that. BBQs and then my B-Day will do it lol. However I signed myself up for the Disney 5k and 10k to force myself to stick with it. I was going to go and spend all that money to simply fail. So I've kept with it. I am going to be able to fully run the 5k but the 10k I will walk and run most of it.
The Idea for me is that when I go for the 5/10k in Nov I will be completed with the 5k training and a week or two into the 10k training. I figure I finish out the 10k training before the end of the year and just run 10ks 3 times a week. Since it will be cold and stuff here in the NE I will do this inside . Once it gets nice I will then start training for the half which is 13.1 miles. But by then I will be running 6.2 miles for a few months . I will then have from around May/ June till Nov to up my mileage for the Half. I also plan to be another 50lbs less for the Half (250lbs).
But I am worried about injuries and I have an older cousin who runs a lot . He is actually doing the challenge at Disney which is the 5k,10k ,Half one day after the other. So he has been giving advice of what to do and what to avoid and so far so good. If I can't' finish the run for that day of the program I drop back to the day before and run that the next time. Then I try again to pass the day I had problems with. So far so good. I also know if I get a pain that isn't normal to stop. I've fucked my body up enough at this point
I would also recommend a low carb diet (paleo diet bordering on ketogenic diet) to speedup the initial weight loss.