Today was better. I wasn't sure what I wanted to do and in the end plumped for an easy 30 minute Zwift ride. That felt okay so I gave it an hour and tried an interval session on the treadmill. I followed one of the Zwift workouts; warmup followed by repeated 40s steady, 30s faster. The faster section got progressively faster pace while the steady pace remained constant so it was run without a normal recovery section. My heart rate slowly increased throughout but didn't top out so all in all much better than yesterday.