The fact that you are already doing 60 miles with a 'total newbie to the bike and.... NOT athletic' stoker is quite an accomplishment - on her part! My stoker/girlfriend is good for 60 miles max after almost 9 months of conditioning. An hour and a half to two hours of riding are more like the norm.
All of the suggestions posted are good. It sounds to me like her reach may be a little long and/or she is riding with locked elbows. I would like to add that if she did some core strengthening workouts off the bike, it might help also. If her torso were stronger, she might have better posture on the bike.
Last edited by galen_52657; 09-14-05 at 11:20 AM.