A big guy can run marathons and ultras! Over the past 13 years, I've run 74 ultramarathons, mostly trail, mostly 50k's (31 miles), but a few 50 milers too. And a dozen or so marathons. My average weight = 240 lbs. for those races. I started with a heel-strike running stride, but, after several years, switched to a mid-foot strike (sorta like "Pose Method" or "Chi Running") which is easier on my body. And, yes, I was the heaviest finisher in each of the 74 ultras I have completed. I attribute my ability to run ultra at my age (51) to the stout musculature resulting from tens of thousands of miles bicycling between ages 16 and 30.

Good luck, and advise if I can provide any help off-list.