Cross Country Skiing
Cross training doesn’t get much better than this: high altitude training, whole body workout, low impact, and beautiful terrain. The motion of cross country skiing, both classic and skating, is great for strengthening of the gluteals. Strengthening of the gluteals has been shown to improve knee pain and IT Band Syndrome.
The research available on cross country skiing has demonstrated similar improvements in the ability to use oxygen (VO2 Max) compared to running. I have not been able to find a study that has investigated the effect of cross country skiing on running performance (race times).