Unable to complete my run today On Saturday, I had my second triathlon (modified sprint distance, age group win) and the ~4.5 mile run was no problem. My weekly long runs are around 6 miles right now, and my mid week runs are usually around 4 miles. This morning I was going to do an easy 3 mile run, and I couldn't get through it without walking. Ate a cup of Greek yogurt half an hour before the run, easy pace, breathing was fine, but my legs were very tired (not sore, just felt heavy). This is the first time this has happened to me, any idea what caused this?