I bike; swim; run and weight train. I've been off running since the temps dropped a lot in my area and since the outdoor track got covered with snow. for 2 years I ran 2-3 miles every day at lunch (weekends off) walking and running uses complimentary muscles to biking. not sure running will help your centuries though. maybe there was something else to consider? sleep? nutrition? nutrition timing? over training? faster stronger riding partner that pushed you a tiny bit past your comfort level?