Postscript

(Just looked at the numbers for the run - as I said, failed to turn on Garmin until some minutes in. 1.76 miles, avg pace 11:28, avg HR 167, max HR 175 - that's 7:18 in zone 4 and 11:26 in zone 5, not surprising it felt so intense! Running in this climate at this time of the year is basically just a highly counterintuitive thing to do, really I would be running something like 9:30 miles at current fitness under the same sort of race circumstance in more moderate conditions...)