I've recently discovered that you tend to use different muscle groups for running and cycling. When much younger, I ran on the cross-country team and could run 5k in the15 minute range. Fast forward many years and I've biked many miles and run very few. This winter, I've been doing some training for a triathlon next august, and my running is stinking the joint up. I am barely able to keep 7 minute miles going, but it isn't my aerobic fitness that's limiting, it is the running specific muscles.