If you're trying to build muscle with 5x5s, marathon may be a paradoxical goal. Bulking up slows you down, and going long burns your bulk.
My particular experience: 20 miles used to be no big deal to me early in college. I was a stick. Later in college, I started trying to balance myself out and get some upper body strength. I'm probably 35 pounds heavier now than when I ran a marathon at 19. I can still go out and run ten miles, but much more than that is pushing it unless I change up my routine to focus more on running. I've got friends (who lift a lot more than I do) telling me that I should cut out my runs altogether if I'm serious about getting stronger.
Now, I'm sure it's not impossible to do both. But at the very least, you're going to spend a ton of time training, and you're going to need to eat like a mofo.
Half marathon is a much more realistic goal, as I'm convinced anyone, even big dudes, can go that distance if they put in the miles. If you don't have a time goal, I'd say just get out and run at least 3-4 times a week. It's a tough summer to do it. Particularly on hot days or days when I don't feel like running, I just travel straight out in one direction for 30 minutes or so, and that way I can't cut corners coming back when I'm wiped.