Jack makes an important point late in this video around the 1:52 mark about the dangers of cross training when you’re injured. Cross training is good to incorporate within a running routine and also as a means of conditioning or rehab if you suffer a running injury. The risk is when runners come back from that injury after cross training like crazy to avoid losing a lot of fitness. All that time you’re stressing your heart but not your legs like they would be when running. So, many athletes quickly get hurt again because they come back too fast. Basically, you’ve got to let your legs catch up again!