Hi. I m an avid runner who ran 2 marathons this summer (3 weeks apart). I began training for another marathon a couple of months ago but stopped because I was tired of running so much. For the last couple of weeks I ve noticed a one of my legs giving out or losing stride/gait after about 5 miles. After that point I cant keep my form because my leg won t cooperate. It always starts with the right leg and then sometimes they both won t cooperate. While I was training I wasn t having this problem and now it happens whenever I run more than 4 miles or try any speed work.