I've had plantar fasciitis before. What it feels like, is a pain in the heel when you put your weight on it. It's caused by overuse, weight or age.
I haven't overused my foot. I do nearly all my caching on the bike, so the feet aren't weight-bearing while I'm moving from cache to cache. I've lost weight (I'm down to 15 stone, 5 pounds on last weighing). But I have to admit, I'm older than I was yesterday.
I've had it before, in my right foot. I can move around the house, but going for a long (more than a mile) walk is definitely contra-indicated right now. But I know how to fix it. I have a method that's worked twice before, and here's the theory.
According to Wiki, "Individuals with plantar fasciitis often report their symptoms are most
intense during their first steps after getting out of bed or after
prolonged periods of sitting." and that's exactly what I get. Further internet research told me more about what's going on here. When you rest, healing starts. While you're asleep, things mend themselves. But while you're asleep, your foot is in the relaxed pointed-down position, so the healing happens in that way. When you stand on the foot, it's now at the 90-degree position, and all that fine healing-up is undone.
The answer is a night splint. Weight reduction is also a good idea, but I'm already trying to do that. There's other treatments including surgery (I don't fancy that) and Botulinum Toxin (no chance). I tried stretching exercises, but they didn't seem to help. But in the past, when I've had this problem, the night splint fixed it. And it's the sort of treatment that doesn't sound like it could be excessively unpleasant.
I have a night splint, it looks like this:
It holds my foot in the "standing up" position while I'm asleep. It's fairly easy to put on, the straps adjust with velcro. It holds my foot in the right-angle position overnight, and the only downside is that if I need to get up in the middle of the night to use the loo, it's a bit of a hobble to get there, or else I take it off and then have to put it on again, all while I'm half-asleep. So, no big downside, and in the past, it's fixed the problem.