Runners who try different distance races quickly discover that every distance has its own unique challenges. My proudest Personal Record is my 10 km at the September 2010 Terry Fox Run, not just because I take that specific race very personally, but also because cutting three minutes off that distance was incredibly hard. My previous best of 52 minutes involved running fast but with a bit of reserve. I held nothing back this time. And I was lighter and fitter than I've been in a very long time. One of my running heroes, local superstar George Aiken, was at the finish line and congratulated me, which made it even sweeter. George is a 5 km and 10 km specialist, and when I told him it was a PR for me he said most people don't appreciate how tough it is to shave seconds off a distance that you've given your all to previously. (George does trails now in his "old age". I don't know what his 5 km time was on Terry Fox day, but he's fast as hell still, and only in his early 50s).
You aren't merely covering a specific distance fast; you're training to a specific pace, trying to achieve consistent speed that will get you over the finish line without collapsing. Well, at least without collapsing every time. Seems obvious, but until you train to a distance you can't fully appreciate the difference. However, looking at the body types of the elites at different distances gives you a strong hint as to what you're working toward.
I have immense respect for marathoners and ultra runners, 100 metre types, etc. Committing to focused training at whatever your distance is takes a lot of effort. And you start to slowly master your distance. For example, after five years I'm finding that I have better control of a marathon than I ever did.
I still have a helluva long way to go to truly master the 42.2 distance but I'm constantly learning and getting better. I have a much better understanding of what it would take to make significant progress in speed. Life, however, requires balance such that I can't commit what it would take to do that quickly. And if I were looking to master the 10 km distance, to truly master it to the best of my ability, the amount and intensity of training would require a huge effort as well.
So I run for fun and I try to go faster.