After watching the NCAA cross country championship races this past year, I regret two things: That I can no longer be a part of a team, and that I didn't know more about the science of running when I was.
I'm older than most on this board, so training back in the 60s and early 70s was to just do what the coach told us to do. All of my training runs were very hard, even the easy runs were faster than what would be considered tempo runs. We raced on cinder or asphalt tracks and our spikes were nothing more than foot gloves. Not conducive to fast times! I wish I were training and racing today.
Watching the NCAAs, I realized how much I miss being able to contribute to a good team. When you're in school, you don't realize that you'll never have that opportunity again once you're out of school (unless you're an elite runner). I wish I had the chance to go back and give it a 100% effort throughout my school and college years.