A challenge for every runner is to decide what forms of cross training best compliment your training. I used to enjoy indoor rowing the day before a long run - It works your arms and legs and gives that burst of energy that fuels into the next day. Yoga and stretching can also help. I had lunch with my friend Kathryn today, and although she is not a runner (currently!), her exercise regimen consists of jump roping, which I believe, is also a valuable cross training exercise for runners. In running magazines and websites, I've noticed that plyometric exercises such as jumping, leaping, "grapevining", and any movement that requires coordination and strength, will help with speed.
Kathryn jumpropes 20 minutes or so each session interpersed with quick periods of walking in place. I used to jump rope as a quick cardiovascular activity in between weight training, usually about 200 jumps in total. I always thought of it as a a compliment to a larger program. When jumproping is brought as the focus, however, it is a powerful pop of strength that additionally puts focus on the arms and also on the balance required not to trip over the rope.
Isn't it interesting that jumproping is usually associated with grade school girls during recess? But that's where we can glean its benefits. Just consider the high degree of coordination required for jumping rope, and especially double dutch, a feat I could never accomplish. Out of curiousity, I checked to see whther jumping rope is actually done competitively. It turns out that many established organizations promote it as an athletic sport. This includes the American Double Dutch League and the International Rope Skipping Federation. As far as cross training activities, it is inexpensive, easy and you can fit it into any portion of your schedule. Jump to it!