If you think that telomere length is a secondary marker of aging and health, then it makes perfect sense that exercise would lengthen telomeres. There is ample evidence to show that average telomere length, while declining with age, is somewhat dynamic in response to circumstances, at least in the white blood cells examined by most present day research: "Leukocyte telomere length (LTL) is a potential indicator of cellular aging; however, its relation to physical activity and sedentary behavior is unclear. The authors examined cross-sectionally associations among activity, sedentary behavior, and LTL among 7,813 women aged 43-70 years in the Nurses' Health Study. Participants self-reported activity by questionnaire in 1988 and 1992 and sedentary behavior in 1992. Telomere length in peripheral blood leukocytes, collected in 1989-1990, was measured by quantitative polymerase chain reaction. ... For total activity, moderately or highly active women had a 0.07-standard deviation (SD) increase in LTL compared with those least active. Greater moderate- or vigorous-intensity activity was also associated with increased LTL ... Associations remained after adjustment for body mass index. ... Although associations were modest, these findings suggest that even moderate amounts of activity may be associated with longer telomeres, warranting further investigation in large prospective studies."