NYTime’s Liz Robbins recommends mixing things up…
Cycling is the best cross-training sport for runners, Mr. Cook believes, since it builds an aerobic base while maintaining range of motion. “It forces your leg muscles to contract and increases the blood flow,” he said. “If you do it properly, you’re not really straining on the bike, the muscles aren’t getting that abused.” Cyclists tend to work opposing muscles — like the quadriceps and hamstrings — differently than they do when on foot. Indeed, recreational athletes who only run can end up with muscular imbalances, according to Bill Pierce, the director of the Furman Institute of Running and Scientific Training in Greenville, S.C. – from NYTimes