See also this reddit thread for details: https://www.reddit.com/r/GarminFenix/comments/1ip4fh4/new_firmware_recovery_time_bug/
I am not the only one experiencing this.
I did a similar run two days before the update with a recovery time of just 24h. Something is off.
HR sensor is broken and frequently measurements are out of... It's not unique for this update, I've seen bigger anomalies in previous versions. fe. a while ago I did light workout with dumbbells and it was showing HR158 i seriously get concerned and put chest strap on and it turned out that it's actually 108 
All metrics are related to HR... I was never interested much HR zones as I believed that watch knows better...but at some point according watch I had almost no training effects and no recovery required despite I've trained really hard, so I have checked those zones and they were set to sky high. My max HR was at least 20 higher than what is possible at my age. I would die before reaching zone 4. Reset HR zones + manual setting max HR to something potentially realistic brought me to more expected outcomes, though it's still far from reliable. Frankly speaking all this watch does currently is showing time, more or less accurately counting steps and logging time and distance of your activities. Rest is just irrelevant mess. While one would expect a serious approach to flagship watch support it's completely the opposite in case of Garmin. I was so happy with FR255 that I decided to upgrade to F7p, sadly it was expensive downgrade. It's my last garmin device.
