Sorry for the panic…
The last tweet referred to ideas of quitting. I now realise that my physical condition was dur to over-exertion. I’d discovered a new (to me) technique for pedaling which gave a dramatic increase in speed. I happily used this to thrash my way up several hills and forgot about it. When I came to the next set of hills a couple of days later, I’d forgotten all about that extra work, but my muscles hadn’t!
I’ve taken it easy for a couple of days and things seem fine now…