None of the serious runners I know do exclusively leg or lower body work, most of them do total body work and compound exercises since overall body strength is critical to good running performance. As a track and cross country coach for women I had them do total body workouts three days a week as the first part of their workouts without any degradation of their running.
In training road runners and marathoners I follow a hard, easy, medium running day sequence with the strength training falling on the easy days which are normally longer steady state runs. Three days on and one day off is the normal sequence however some prefer to do six and one day off.
Bottom line strength training improves your ability to run. www.discoverstrength.com/training.php
As a more specific answer do your strength training as total body and not leg work and do it on days you do not run.
Edited by: SERGEANTMAJOR at: 3/2/2010 (00:39)