When I had the urge to go for a long bike ride, I generally did it...and suffered the PEM afterwards. Sometimes my legs would feel up to it, but my brain just wanted to flop down on a pillow, so I didn't push myself in those circumstances. The point was that I used to have those urges. In previous years, I should have felt like hopping on my bike by now and seeing what was new along the roads. I should also have felt like going for hikes through the woods. Those feelings are simply missing. I did find the motivation to walk around for hours spraying roundup on thistles, so it's not a matter of physical limitations. I'm sure that I could go for a bike ride of at least a few km (prime limitation being saddle acclimatization), but I just don't feel the expected desire to do so. Likewise for hikes: I should want to see how the wild blueberries are doing or how the beaver ponds are doing, as I did in previous years, but thinking about it now just doesn't make me want to pull on my boots.
As for minor projects, I'm talking about low physical activity ones, such as soldering some replacement transistors into a power inverter. It's not really much time or effort, but I lack the desire to start. A more rewarding project would be to build a decent metal detector. I have the parts and materials and plans from when I did feel motivation, so it's not a major undertaking, and the results could be rewarding (fun!), but thinking about starting it triggers no desire, which I would have felt in previous years.
Last year I managed to build an outdoor clay oven. This year I did manage to build a couple of birdhouses (which I did the previous year too). I think I managed the latter project because of the time factor: build it now before the swallows arrive, or regret not having done so. The rides or hikes have no such motivators.
The link about unrewarding rewards is interesting, and I could see it applying to the long bike rides, but not to short walks in the woods or sitting at a desk soldering parts.
My guess is that it's just part of the complex changes in neurochemistry involved with CFS. I posted it on the chance that others would respond: 'Hey, now that you mention it, I realize that mine has been slipping away too.'