About 6 weeks before the Chicago Marathon, I ran a half marathon at my intended marathon pace (8 min/mile). I wore my HM and found that I needed to clock about 158-160 bpm to do it. It was remarkably regular and accurate.
When it came to Chicago, I found I was running nearer 168-170 most of the way, and I only did 8.15 min/mile. I seemed to be perfectly healthy (no temperature or anything) but it was exceptionally cold all the way round.
Does anyone have an explanation?