Hi M.,
Congratulations on your second child. The fact that you are already wanting to exercise, along with your midwife's permission, is a testament to how well you are doing. As a perinatal fitness trainer, I normally have new moms get the okay from their doctor/midwife after their six-week check up. This is typically the desired amount of time for a healthy new mom to wait before getting serious about exercising after birth.
I encourage you to take it slow and, although it sounds like you have the energy, here is a link to an article I wrote for new moms and exercise. If you are wanting to get back on the treadmill, this breathing exercise will help strengthen your abs, being mindful of posture as you walk/run.
http://www.citibabes.com/assets/citibabes/articles/0000/1...
Begin slowly and pay attention to your body so you don't push too hard, especially in the beginning, and good luck!
S. Caesar
Founder
Maternal Dance/Rhythm For Life
www.maternaldance.com