Correct me if I'm wrong, but don't sprinters usually have bigger legs while marathoners have slimmer legs/mass? I guess it all depends on the OP's ultimate goal, which I believe is to go "further", so I'm not sure if mass is what you're looking for exactly. However, a weight training program aimed at higher repetitions, along with super sets, and plyometrics would be my recommendation. Conditioning is all you need kid.
I'm sure others will object, but that's my 2 cents.
This is also why most MMA fighters are built lean rather than bulky(of course you have the exceptions, but are usually bulky from their height and stocky build). They strength train for endurance rather than bursts of activity then periods of rest in between. This is the difference between football/basketball players too....Football they have to explode into plays, but then have rest in between plays, basketball has way more running along with jumping so they're built for endurance along w/ explosive strength.