Can you use an exercise DVD during the week? Borrow some from the library to find a few you like, then buy them if you can commit to that. Sometimes kids like to do stuff that's on TV! Also look into some parent/child exercise programs for the TV or the Wii if you have one.
Do you like to bike ride? I'm not sure how expensive those little trailers are to put your daughter behind you and tow her along. She might be a little big for the kiddie seat you put behind you but that's another option. Sometimes you can find these things at yard sales.
Can you look into play groups in the area, either through the library or something like the Newcomers Club? You might meet another mom or two, hit it off, and then you can watch each other's kids while you work out or go for a jog. Your daughter can socialize and have a play date, and you can have some free time. I don't know what your relationship is with her dad, but would he enjoy a few hours with her during your custody time so that you can work out?
Good luck. Just the fact that you are working on yourself is a great thing! DO take time for yourself. You deserve it, and your daughter deserves a mom who is happier.