At her age, I'd probably go with formula if you're going to replacing a feed instead of cow's milk. Pumping is tricky, as our babies get older, it gets harder to start pumping at a certain time -- our bodies get used to producing at certain times and not others. That's not to say if you kept up pumping at a new time, that your body wouldn't be able to produce more. More pumping sessions would certainly increase your supply, also making sure that you pump for 15-20 minutes, even if the milk has started flowing. Pumping and then pumping 20 minutes later also helps some ladies with supply.
The WHO recommends 4 milk feeds per day until a year, that being said, I have heard several ladies mention that they went down to 3 somewhere between 9-12 months so you're not all that out of line. You also mention that you've started an exercise class -- are you sure that you're getting enough water? Or enough calories? Or enough rest? All of those things can have an impact on supply, it could be that if you're trying to pump when you get home that your body is in need of some more water.
HTH!