Hi J.! I agree that the uterus does not shrink as quickly after the second, that was my experience too. It really took me a good year to feel somewhat back to "normal" after my second, but that was not the normal I had before I had any kids! I find that while the scale may say the same thing as it did before having my kids, things have just fallen and settled a little differently now! Anyway, I started by just walking with the stroller to get a little resistance and a better workout. That worked really well! I live in a very hilly area so that was a little bonus. I am a runner, so after a couple weeks of walking I was ready for jogging, then I built back up to a decent run. It honestly took about two months of that stage until I started to feel my breathing settle back into the usual rhythm, and I didn't feel so winded (I also had c-sections, so the recovery was longer and harder). You don't need a gym for either of those options, though I do belong to a gym and I was lifting weights twice a week and doing a circuit routine to try to get the metabolism back up and swinging. You can, however, do this at home, by incorporating jumping jacks, mountain climbers/running man, squats, high jumps, etc., and you could buy a set of 5-lb. and 8-lb. dumbbells and do this sort of thing in your living room. For women it is important to do a little strength training but to try and keep your hear rate up, not just stand there and do a few bicep curls. It is the best way to burn fat.
None of my working out affected my milk production. I limited it to no more than an hour a day, sometimes less, because I did feel very "full" by the time I was done and it got a little uncomfortable. Plus, the baby would usually be ready to eat again at that point. It got a lot easier to predict my kids' eating schedules when they hit the 2 - 3 month mark, so I'd plan accordingly, Good for you for making this effort! I am not a woman for whom weight just naturally falls off, I have to work at it, but I have come to savor my workouts and I feel so great afterwards. Good luck!