I had a question and hopefully someone has some insight. I run as well as bike (no triathlons or anything though), and I recently started to have a problem. I am currently training for a half marathon, and in the meantime I have continued biking (commuting, getting around the city) as always. The problem is, it seems when I run a day after biking, I am noticably slower - even when I only biked 5-10 miles. Does anyone know how biking and running affect each other in terms of muscle structure? Is this mental, or is biking really making my running more difficult?