We all do things because we're supposed to be able to, because we were "born to do it."
I know a lot of runners who were accomplished cross-country runners in high school who have failed in their attempts to qualify for the Boston marathon. Some of them fell out of shape over the college and post-college years. Some of them never made the full commitment to train for a marathon versus shorter races. Some of them just hit upon misfortune during their attempts.
I am not such a runner. My high school fitness test "mile of running" was north of 13 minutes (a six hour marathon time limit is a 13'44" pace). In fact, I never *ran* a consecutive mile until 2006. That mile is still one of the hardest I've run in my life.
After training for my first 5k for 8 weeks, I ran my first 5k at 206 pounds in 33:44--a 10'52" pace. With that sort of 5k time, McMillan running predicts a 5:30 marathon.
Today, I'm at 168 pounds, and my fastest marathon time was 15 months ago in 3:39 and change. Since then, my speed has improved quite a bit, but my endurance has suffered through injuries resulting from trying to push too far, too fast. The race predictors put my current 20:59 5k time as capable of running a 3:20-3:30 marathon (with proper training.)
Until the day I turn 35, qualifying for Boston requires a finish time of 3:10:59 or faster. Up until 40, I still have to beat 3:15:59. My current training plan puts me at 3:17.
To put it simply, I'm not capable of qualifying for Boston. There it is. I'm not good enough.
That's why I'm training to qualify, by 40. It's because I wasn't born to do it. What good is life if you meet everyone's expectations all the time, anyway?