Disappointed in the battery drain during my marathon...

I ran the Wineglass Marathon this past Sunday.  I made sure my watch (645 non-M, software version 6.2) was fully charged beforehand, and turned off the bluetooth.  When I looked at the watch at mile 10 (about 1:45 into the run), the battery was down to 60%!!  Yikes, that would mean a dead battery before the end of the run.  I wound up changing the GPS from GPS+Galileo to just GPS, and turning off the heart rate monitor.   I made it to the end in 4:40, and had 10% battery life left.  It was definitely a distraction and concern during the race.  I have the NYC Marathon coming up in 4 weeks.  I'm guessing I'll just leave the HRM off right from the start.  But very disappointing. I did two marathons with this watch last year and had no problems with battery life.