Hey congrats on your new addition, lucky you!
If I were in your situation I would start from scratch as you would an untrained horse. What this does is give your mare a refresher course & you get a feel of what she can do & how she reacts. This will give you a great foundation start.
I'm sure things will go quite quickly depending on how well she was trained & her temperament. Some horses resent going back into work because the *leisure* life was a great place to be

and some horses get back into it with such a great attitude, you'd swear they never took a break.
Now having said that, if your mare is like the latter, be careful not to push to soon, even though she seems like you could just carry on. You need to remember that after having babies, her muscles are lax (soft), so little sessions either on the lunge, long reining or under saddle should be kept short. 20 to 30 min at a time. Lots of walk for the first week, add some trotting the second week. I would leave canter (esp. under saddle) for a few weeks because you want her to rebuild her top line muscles first.
After the second week you can probably either gradually add 10 min increments to the sesions or add hill work (if possible where you are).
By the 5th week she should begin to seem fitter & begin having more stamina.
Just remember to take your time; she'll appreciate that.
Hope this helped.
I'll be bringing back an older mare back in training as well. She's been off for 10yrs now.
