I was rocking an awesome battery before updating to iOS 4: a full two days including two 45 minute workouts each day. Now I'm down to 52% and I've only worked out one time and I've been wearing the watch for 6 hours!!!!
My SS AW2 has been on since 7AM. It is now 1:17PM and I am still at 90%. Updated yesterday and didn't have any problems. Not sure why yours is acting up.
My morning workout was only a walk, but I ran with it yesterday evening.
If you just installed it, give it sometime. The OS is doing background work like indexing. This eats up a lot of battery until it completes. It should begin to stabilize within 24 hours.