Many of us can't run five days a week. At least not forever, especially after injury creep sets in upon departing our 30s. Not everyone can handle that steady pounding on hips, knees and feet.

If you're training for a longer running race, some experts advise running three days a week (speed, hills, long run) and filling out the other two days with cross-training. Cycling is a good cross-training activity for runners -- it's aerobic, works opposing muscles (hams/quads) differently than the running stride, and 90 rpm on the bike matches the average runner's cadence. A runner who skips cross-training can develop serious muscle imbalances, resulting in injury.

Kristen says yoga for runners is more important than you might think, too. And swimming offers cardio with nearly zero impact. If you're injured, try deep-water pool running to keep you in stride while healing. To runners everywhere: Cross-train now to stay running. Your body will thank you.