Running is lower body, and the muscles are usually long and lean, and it is an impact exercise. Plus, the muscles in the front of the thigh are usually strengthened a lot. In horseback riding the inner thigh muscles are worked. The horse is doing more work than the person riding, but it does take physical athletes and stamina when riding for sport. Swimming is whole body with no impact. It requires a lot of stamina like running. Long distance is different than sprints, and swimming in a pool is different than a natural body of water.