AlexR, I'm not especially heavy- 89 kg and 184 centimetres and quite lean. But I look more like a javelin thrower or a rower than an endurance runner and am much bigger than most people in my club. It's of absolutely no use to me in terms of running to have a relatively powerful upper body, and I haven't done anything (like used a gym) to gain this.
It obviously does matter to some extent if the mass is lean or fat, but either way, your heart and lungs aren't proportionally much larger with weight. So it's always going to be a struggle to be a fast endurance runner if you are on the heavy side! Your energy output per mile is going to need to be higher, and the longer the event, the more that 's going to be a factor. Look at the build of fast (elite) endurance runners and there are no exceptions whatever to the ideal build archetype.
It most definitely is worth running, whatever your weight, but do be realistic about what is possible in terms of pace over the longer distances. Does that make sense? I've just done an off-road marathon and loved it.
Edited: 29/10/2012 at 16:36