When to excercise?

I woke up this morning, looked in the mirror, and decided I really do need to excercise. Not that I'm obese or anything, just flabby. It's really driving me crazy. My question to myself is this, "when do I excercise?" My husband leaves for work at 5:30 in the morning. The kids come at 6:30. The problem isn't that I'm not willing to get up at 6:30, it's that all the rooms are taken by sleeping children. By the time they wake up, it's time to get my 2nd grader ready for school and my day begins. Night time rolls around, hubby doesn't get home till 7:00. Homework has to be done, baths given, and kids put to bed. By that time I am so exhausted, excersing is the last thing I want to do. Priorities. That is what it all boils down to I'm sure. I've never LIKED to excercise, but I think I'm at a point in my life where I either do it, or get fat. That doesn't sound like a good option to me. I keep telling myself 20 minutes a day. When to take that 20 minutes--that is the question.