Sometimes when I'm running I feel great. Other times, I feel like I have lead in my shoes. It seems to happen completely randomly. The way I deal with the down days is to still go running, still try to get my mileage in, but only allow myself to focus 50-100 yards. I pick a target and run to it. When I reach it, I pick another target and run to that. I've gotten through many hard miles that way.
In your situation, FreelanceWanderer, maybe you could set
Microtasks
and do a little bit of work at a time. You may not be AS productive as on a normal day, but you'll keep moving forward.