I am on the 10th week of P90X and I really believe that doing the program has made me both quicker and have better endurance as a runner.  I started the program for the first time last November and my husband and I got a little over half way through when we went away for Christmas, then for vacation and life....so we didn't finish but have been doing it off and on throughout the year.  We started again this fall and are determined to finish the 90 days in a row!  I just ran the Chicago Marathon in October and took 14 minutes off my time from the other marathon I ran and also qualified for Boston in the Spring!!  It is time consuming though but definitely worth it!