I'm struggling with this concept too hun! With DS still an average (ok poor) sleeper, early morning or late night doesn't really work as he's usually up anyway. I think I will have to try going for a power walk (and build up to jogging) on weekends when DH can watch the kids - if I feed DS and leave straight away that would work.
I've contemplated doing a gym class when DD is at daycare but that means putting DS in a crèche (and he's still so little!) plus having to be ready at a set time. We do have yoga class but its not much of a workout so I should at least take DS for a power walk in the pram on the two daycare days...