Exactly. You can't just "sneak up" on a higher average speed. (given infinite time, maybe) When I decided to see if I could actually break my personal "5-minute mile barrier," (running, thank you very much), I spent a lot of time doing speed work in 1/8's and quarters at the 5-min. pace or better. After a lot of work, the segments merged into a 4:57 mile. I am convinced I would never have gotten there by just "trying harder" over and over again at the whole distance.

Tyson