I have similar issues with PV. It’s been a few years. I have opted not to take any medication other than baby aspirin. My platelets are stable at 700 and WBC at 18. I have fatigue and weakness all of the time. However, I have found like you, I’d I do early morning, moderate exercise I feel much better. And throughout the day, I will stop my work or resting, or whatever I am doing, and do a short exercise/cardio video and my oxygen level remains at 98% and heart rate 75-80. I am a,ways reinvigorated. I say moderate as I am cautious in moving. Slower pace, 15-20 minutes of movement and The energy level rises for a few hours. I had a similar thought as you, maybe endorphins …..maybe depression or sadness causing the tiredness/ fatigue. Whatever, just keep doing what I can. Volunteering to help and staying in touch with friends and family. I pray a lot and have been better at letting go and letting God take a hold of me…..also, I do research dietary issues that might help as well.
Gods blessings to you.