I Am A Long Distance Runner, I Have Ran Marathons

I am a long distance runner, I have ran marathons and regularly run 15+ miles, however over the last 2-3 months I have not had the energy to run and I can barely run 4 miles now. It is not aerobic, i don't have trouble breathing, it's like whatever chemical reaction needs to happen to put energy into muscles isn't happening. I am not overly tired otherwise and do not have any other conditions or changes in lifestyle. I'm wondering what might be causing this?