You can up your exercise without needing much time! Do a bit of walking - to the next bus stop if you use the bus, parking further away if you drive.
When the kids are in bed you can change into your trackie bottoms and go out, walk briskly for 5, 10, 15 or 20 minutes to speed up your metabolism and heart rate, and then be back home inside 30 minutes. Once you've been doing this for a while, you may want to do Couch25K, which many Mumsnetters have used with great success.
If you don't have a DP/DH at home to enable you to go out for a short walk, you could get a rebounder (small trampoline) and start gently with five minutes or so, building up to longer sessions. You can do this in front of the TV. Argos have a fab Reebok one for about 30 quid, or look on Gumtree or the local paper.
I started with a rebounder while I was using Foodfocus, and bounced for about three months before starting couch25k. I'm now hooked on running, as I love having that 30 minutes on my own to clear my head in the evening.
You don't have to think big. Even a tiny bit more exercise will help.