Note the tense: "Was".
I thought I'd check the pressure in the tyres, maybe pump them up a bit.
When I removed the pump from the rear wheel I managed to remove the valve from the tube at the same time...
Replacing the tube cost me half an hour and 3 sessions of solo riding. Sigh.
Still, ~90Ks later I'm still on track to get to Mt Gambier on time.
More later I hope.