After another physiotherapy session, it's become quite clear what I did wrong. I failed to respect the stretch.
My right calf muscle is plenty strong, but it's tight and now it's hurt. I kept increasing my distance, but I didn't stretch before or after. I knew I should, but I thought I could get away with skipping this step. I was wrong.
Now I'm stretching my calves all the time, trying to ease my way back. I'll try a light run tonight, just for he heck of it. I'll never run again without stretching beforehand and afterwards.
If I can save just one runner from the same fate as me, my pain will have been worthwhile.