M.R.
Sounds like a good start. I lost about 4 sizes breastfeeding my second child and was my smallest adult size. I started working out again at 6 weeks (I hadn't done that regularly for a long time) and worked my way up to about 30 minutes of cardio a day 4 or 5 days a week and walking with the kids on the weekends, etc. I also ate pretty healthfully and tried to get between 1500 and 1600 calories a day. Your body needs between 500 and 600 for full-time nursing, so if you take that away you still need to eat enough for you. :)
I did make sure to treat myself to things once in a while, I kept dark chocolate pieces at work and tried to bring almonds to snack on. I find that if I set my food out ahead of time and take my days' worth to work I stick with it pretty well. When it's gone it's gone, as long as I bring enough. Oh, I also pretty much gave up soda but do treat myself to a real Coke once in a while when I really need it. I don't like any "lite" versions of foods with the artificial sweeteners. I don't know enough about them but they sound nasty. Good luck!