Have you tried Bikram Yoga? (That's the yoga in a hot room.) Its definitely aerobic. Its also suitable for all fitness levels, as if you're not too fit/flexible, you just do as much as you can and no more. I know its not for everyone, but it really helped me both physically and mentally. Only problem is that classes are 90 mins and most studios that I've been to don't have childcare, so you'd have to find a babysitter. Still, if you can find someone to watch your bub for a couple of hours it might be a good thing to try.