I'm a single mother, so I know what you mean.
I've find it hard at times. Right now I want to eat. I think it's just easy to give in when you're caring for children. So, I would say be prepared for that.
I just read a good post on this site about how to keep going, you might want to scroll down and find it.
I would say, try to plan some activities ahead of time. things that wear out your child and reserve your energy. Go outside as much as possible. bring a blanket to the park and sit out there with books. get videos....coloring books....
have things organized as much as possible with food for family. so that you don't have to think about it.
give yourself some alone time, bed early, naps if possible, lots of baths!!! they really help.
expect to want to go off. and make a list of reasons you should stay on it. pin it up. look at it. remember.