Two problems surfaced during my last run.
1. I lifted the rear of the bike by lifting the saddle ... and the saddle broke. It was still usable, it was just the rubber bushes that damp the suspension that came apart. But it's still a broken saddle. So I replaced it, from the bike that I bought just for spare parts.
At the same time, I remembered that daughter.1 had bought me a kind of saddle cushion. It isn't a saddle, it's a cushion that fits over the saddle, to make it softer. So I put that on, we'll see how it goes.
2. The inner tube rotates inside the tire, which makes the valve sit at an increasingly sharp angle. Eventually, this sheers off the valve, and the air escapes - this actually happened to me twice. I'd rather it didn't happen again.
To deal with it, I deflate the tube, wiggle it straight, then reinflate it. But why does it happen, and why only the rear wheel?
I was out caching yesterday, and I was idly thinking about this and that (caching gives me an opportunity to think deep thoughts) and I realised what's causing this, and why only the rear wheel.
It's my custom to mostly use the rear brake for slowing down - that's because you're less likely to go into a catastrophic skid that way. The brake acts on the wheel rim, putting torque on it to slow me down, but the tire is still trying to continue on its way, as is the inner tube. And the differential between those forces, it what rotates the inner tube relative to the rim.
So now I understand the cause, I can start to think of a more permanent solution than the deflate-adjust-inflate method.