I know your post is old but I’m desperate. I knew I was heading towards CFS and like you, if I did a tempo, I couldn’t finish the session, then race .. no problem. How are you getting on 3 years later. I have just been diagnosed and I’m currently unable to walk more than half a mile. The thought of never running again is breaking my heart. My OTS was post fast marathon when I hit the wall. This then Jed to CFS. Please help. Is there light at the end of my very dark tunnel?