One of the first pieces of evidence that another sport could improve running came from Tom Miller, M.S., when he was a Ph.D. candidate at the University of Utah. Miller had runners warm up on a stationary bike at low tension. Then they increased the tension until the wheel would barely turn. Next, the athletes stood up on the pedals for two bursts of high power pedalling for 30, 45, 60, 45, and 30 seconds. Between each power burst, the tension was lowered for recovery spells. All runners who used this training method a couple times a week for six weeks improved their 10K times; some even had PRs. High-power bike intervals work your leg muscle even harder than uphill running, but without the impact of hard running.
Even if you're fine with your current endurance level or speed, I see one other important reason for cross-training: the switch up. It's nice to be able to, once in a while, switch to a different activity. I started adding biking as a way to cross-train but also as a way to give my legs some reprieve from a high impact sport. I also did it because variety is good. And the thing is, after a bike ride, I'm SO ready to go back to the sport of my heart the next day. I missed it a little. Pathetic, I know, but it's the truth.
There are other ways to cross-train as well. Among them are weight training and walking. Weight training is to improve your strength and walking to improve endurance. For more info on these and other cross-training news click here.
So give yourself a walk or a bike ride, to switch it up. I used to think doing ANYTHING besides running was a waste of time, time when I could be working on my running muscles. But since researching cross-training, I now know that I can go on a bike ride feeling confident that while I'm leaving the runner at home for the morning, she'll be just fine and she'll be even stronger by the time I get back.