It seems that almost no one understands a few simple things about running:
1) Two runners in the same race may be limited by different things.
A friend of mine, who's training to run a sub-3 marathon, ran a 5K race and discovered that everyone else was breathing much harder than he was and, when he finished poorly, he wondered how guys who were obviously working too hard could be beating him. I told him the other runners were probably wondering why he didn't just run harder - he didn't understand.
When talking about racing a marathon, I mentioned my taking a breath every stride for the last hour and another friend of mine exclaimed that it would kill him to breathe that hard for that long. It wouldn't make sense for him, but it's necessary for me.
When I raced a half-marathon a few years ago, I ran at a heart rate of 178-179 beats per minute for all but the first few minutes, about 98% of my maximum heart rate of 184. According to many experts, this is impossible. Jack Daniels has his runners do interval work at 100% maximal heart rate, but I can only maintain 184 beats per minute for at most two seconds, even though this is very close to the 178 I can do for 90 minutes.
The reason for this is that I am limited by my ability to transport oxygen. To compensate for this, my training has developed a heart that has a high stroke volume and corresponding low resting heart rate, a high maximal heart rate for my age and a high lung capacity - and yet I have to breathe twice as rapidly as many of the runners running beside me in a long race. These runners have a different limitation, so they don't need to breathe as hard and they may lower stroke volumes, lower maximal heart rates, etc., without it being a concern.
A lot of runners have the idea that maximal oxygen uptake is the be-all-and-end-all of training, when it might not be very important for them at all. In my case, it happens to be important at all distances beyond the mile. And I just might be a "natural" miler, where that limitation is not important.
2) One runner may have different limitations in different races.
Heat dissipation is a major concern among marathon runners, but it's not as important in shorter races. Shorter, thinner runners have greater surface area per volume than larger runners, so it's not uncommon to see very small, extremely thin marathon runners. These same runners make poor sprinters, where power generation from large muscle mass is important. While runners (and especially female runners) often obsess about their weight, it may or may not be a limitation.
3) Limiters to performance may change over time.
When I was racing in high school and college, it was not uncommon for someone to tell me, "The least you could do is break a sweat!" It looked like I wasn't trying, because I didn't sweat as much as others and that was because I didn't need to. 10-15 years later, I gained 40 pounds on a very slight frame, some muscle, some fat; this caused greater heat generation (with greater insulation) and I started to sweat as much as other runners. Heat generation had become a factor in my performance.
4) Elite runners have two equal limiters to performance, at least one of which makes them unusual, and what makes them unusual is also what limits them.
Elite runners often are freakish. Some train extraordinary numbers of miles; some train very few. Some are extremely thin; some extremely muscular. What they have in common is that they have pushed against their main performance limitation, often to the point that their bodies have adapted to extremes, right to the point that some other factor also limits them.
Take, for example, Matt Carpenter, who has had the highest recorded maximal oxygen uptake measured among runners (over 90 ml/kg/min). At 5'6" and 118 pounds, his weight has dropped unusually low, which is one reason his VO2max is so high; it also dropped to the point where further weight loss would hamper his ability to run fast. His VO2max is the major reason he has done well in mountainous (and particularly uphill) races, but he is not an elite runner at the same distances on flat ground, where he is limited by other factors, including the very things that gave him the high VO2max.
The takeaway: don't think that losing weight will make you faster, just because you see lighter runners winning races. Don't think that running more miles will make you faster, just because someone who's winning races is running a lot of miles. Don't base your running on someone else's needs, don't follow someone else's plan. Find out what you need to do to reach your goal and then do it.