We don't have a gym around here as we're too rural, but I was back to working out just with long walks, hikes and then eventually back to running within about 2 months or so (though the running wasn't until maybe 4 months, I don't quite remember). For long walks and hikes (2-4 hours), just took baby with me and usually she slept most of the time. As for runs, I did it in the morning or evening when I could find some time to sneak away for an hour. In the early days, babies feed a lot, but by 2-3 months, it's much more regular so it was easier to fit time in for myself. If it's hard to get away, you might find it's easier just to walk or run around home for a bit until your baby goes longer stretches between feeds.