Lol, I can't wait for spring either. I would only do it for an hour at a time if you were worried, but once she is getting worked again, she should be fine and getting more fit. If it is a cold day out, then I would not put the muzzle on, but she won't be starving and it will help her with the weight. Horses don't really stop eating when they are full because they like being full, but when horses have too much food in their stomach it can lead to a bowel obstruction which doesn't happen often, but I have seen it and the horse with unfortunately pass away. My uncle has a farm in Texas and I see it mostly in his cattle but has happened with some horses. Even trotting her for 5 minutes in the pasture will help, but too much grass may cause some problems. If you do put a muzzle on her for a little bit, make sure she gets the hang of drinking water with it on. Just so she doesn't stop drinking.