I get to the gym by 5:00 am so that my workout is done and I'm home before the kids ever open their eyes. I am NOT a morning person so this was a difficult transition for me. Now that I am used to it I really, really miss it if for some reason I don't make it.
Sometimes I choose to go to a night class or take an outdoor walk instead. I used to feel guilty asking DH to watch the kids while I took the time for myself. He didn't mind but I felt like it was my job. I realize now that I am a much better wife and mother if I take that time. Funny thing too, nobody missed me being gone for an hour or so anyway!
I don't work anymore but when I did I kept a couple of dumbells in my office. Sometimes I would close the door and jog/march in place and do a couple of arm exercises while I was catching up on email. I took the stairs and parked at the back of the lot. Every little bit helps!