Thank you, ***** ***** comforting. There is a small back story that may be relevant. Between May 15 and Jan 16, I gradually built up until running 3 times a week for approx 16-18 miles total. In mid January I ran a parkrun 5k in the mud and probably over exterted - ran at about 100% of max HR for the 33 mins. After recovery, a few weeks later after about 1 mile in a run, I suddenly felt very fatigued for no reason and had to walk for the first time in months. (There was no heart rate spiking) It happen again a week later and I assumed a virus or similar and stopped running for a few weeks. Then I went to India and contracted a stomach problem, so ended up having about 8 weeks off running. I started again 3 weeks ago running only a few miles at a time and at very slow pace.