Before his accident, Finny had been a multi-sport natural athlete of the highest degree. While his successes came easily to him and he didn't like to make a show of his abilities, he did enjoy physical activities and did realize that his skills could open up many opportunities for him in the future. The shattering of his leg ended all those dreams.
When Finny returned to Devon, he was horrified to learn that Gene was meeting the required sports involvement by serving as assistant senior crew manager. Finny quickly brought Gene to the realization that Gene was destined to become the athlete that Finny could no longer be.
"Did I ever tell you...that I used to be aiming for the Olympics?...Well, I was. And now I'm not sure, not a hundred per cent sure I'll be completely, you know, in shape by 1944. So I'm going to coach you for them instead."
Finny intended to train for the Olympics vicariously - his participation would come through Gene's involvement and his coaching.
finny trains gene for the olympics, because finny wants gene to do the olympics since finny can't do then anymore