If the battery drains too fast after an update, see all the replies to:
Update 6.2.1 Battery Draining Too Quickly… - Apple Community
Battery Drain since watchos 7.4 - Apple Community
WatchOS 7.5 battery drain - Apple Community
FWIW, in some cases, unpairing the watch, completely erasing it, then pairing and restoring the backup will resolve the battery draining issue after an update. (Simply unpairing and pairing again may not work.)
Unpair and erase your Apple Watch - Apple Support
Also, some people have found that simply waiting a few days after an update resolves the battery draining problem.
As far as the fitness app goes, the first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support