I have one regret.
My biggest regret by far was trying race walking as a cheap shot of getting to the Olympics. I knew I could do well.
However, my knees were not cut out for the stupid action of it. After a few months I tore my meniscus and damaged a load of other structures in my left knee. Ten years on and it still isn't right, it's a permanent injury and in terms of its impact, it's nearing a disability. Some level of pain every day, and some limitations.
I wish I had stuck with running and never tried racewalking, not even once. Aside from that I felt stupid doing that by the side of a road.