you just have to work through the burn sometimes, which is the only way i've dealt with it. Your forearm gets tired, it's as simple as that. it can't keep things up forever no matter how fit you are. and working out generally doesn't work the muscles you use to finger a chick. my preference in moves to stimulate her are to start with the fingering, but only use that to tease and get her going. then i'll move to some tongue action
. for me i have found it easier to stimulate the girls i've been with orally. the trick is not to just lick; suck on the clit and all that as well. do some more playing around and figure out what works best for you though. also as you finger her, you build up the endurance your hand will have for the future, so don't give up