I went through the same thing - I gained about 10-15 pounds once I stopped nursing. You burn over 500 calories a day just from breastfeeding - when you think about how much that is, it is huge! That is almost twice what I used to burn doing my usual workouts.
Once I realized where my body was going, I started an exercise program - 4 days a week of half hour aerobics plus some weight training. I did this for about 5 months and had great results. I lost all the weight and then some, plus I felt toned and fabulous. Then we moved into a new house, and the project of packing, moving and unpacking took up all of my time. It's been hard to resume my routine ever since (that was about 10 months ago). I still work out some every week, but nothing like I used to. But I am happy to say that the weight has not come back. I think once you get rid of that initial gain, your body re-regulates and balances back to your natural weight. At least that's what I keep telling myself!
I do need to get back on the bandwagon, just because I feel so much better about myself and the world when I am exercising. My goal for the summer is to take advantage of the warm weather by getting out there with my girl in her stroller and jogging a few times a week. That is the plan!
Happy training! : )