Thanks for the encouragement, everyone. It worked!
I was resigned to dropping out of the challenge - still not feeling 100%, and lots of other things in the diary over the next couple of days. But I took an afternoon off work today, and went out. Chose an easy route - completely flat, along the coast, with "bail out" options (train) should it all go horribly wrong.
And kept the Garmin in my pocket - didn't want to be looking at speed, or the time, or anything like that. Just a slow, gentle spin along the coast.
Through the town, then followed the A78 for a few miles - sometimes on the road, sometimes on the cycle path. Minor road out to Portencross, stopping for a few mins just to take in the view. Then farm roads and a bridleway to the power station. About 20k.
Didn't fancy heading on to Largs, my usual destination, as the road was a bit busy, so headed home by a similar route. Through the town and along the seafront. Hadn't looked at garmin, guessing about 40k.
Stopped and had a chat with a guy who has just opened a bike repair place in one of the beach huts in Ardrossan. Only been open for a few weeks, but he seems to be busy. Good location, lots of passing bikes on the NCN75.
I was guessing I needed another 10k - and my favourite cafe was 3k away. Had something to eat and a coffee there, then home by a roundabout route.
Looked at the Garmin as I turned into my street- 49.6k - so rode to the other end of the street and back
So I'm in for this month!