I don't have any suggestions, really, because I'm too new to know what might work. However, I do have a couple of ideas ... (maybe that's the same as a suggestion but in my opinion suggestions come from people who know...ideas are brainstorms. hahaha). First, you'll be too tired to eat!
Second, maybe you won't be able to fit in a full blown exercise for the first little while but could you grab 10-15 minutes here and there? What about dancing around the room with your new one. Could be some sweet bonding. Thirdly, incorporate your exercise with playing with your older kids. That way you're not losing time with them but you're still doing something good for you. I love to jump rope with my daughter and play basketball with my older 3. It's summer in Guatemala....maybe you could stick with a lot of fresh fruit? Anyway, just a couple ideas from my 4 child home to yours.