TBH, before my children were at pre-school, it was v hard. When I had DD, I used to do a Davina DVD in the morning when she napped, and then started training for a 5k run once I'd improved my fitness. I would also see a personal trainer once a fortnight or so to keep me going (knowing I was seeing him, kept me running in the meantime to show I hadn't slipped in between sessions).
After I had DS, I can't actually remember what I did, if anything - probably the odd jog as and when I could (ooh, just remembered, bought a Wii fit). Once DS had started pre-school, I had 2 days a week with some free time carved out (the other 3 days I worked). Over the years, I've done quite a lot of jogging (and this is v popular where I live, with several groups going out together - meeting at school gates in kit and going straight from drop off), various classes (really enjoyed Zumba, was lots of fun).
Exercise definitely does not come naturally to me, I'd much rather sit on the sofa after a hard day, but at the moment, I'm seeing a personal trainer on my day off (both DC now at school), going to a Circuits class on one evening (8-9, so after bedtime, plus DH is usually home in time), and doing something active as a family at the weekend (tennis, bike ride, playing football, long walk). I look the best I've done in years, I have more body confidence, and I have more energy. Plus I'm modelling good behaviour for the children.
As you say, the hardest thing is getting out the door, but you'll never regret it once you're out. I find having a date in my diary (personal training, class with friends) means I go. Having something general means I can find an excuse (and I will. I'm great at that).