Modest exercise gives maximum health benefits.

Gretchen Reynolds points to a study in the Mayo Clinic Proceedings that reviews studies published in PubMed since 2000 that included at least 500 runners and 5-year follow-up to analyze the relationship between running, cardiovascular disease, and all-cause mortality. The optimal dose of running required for protection is surprisingly small. Running for 20-30 minutes twice per week appears to give maximum benefits. Three to four times the duration of walking is needed to achieve the same benefits.