Not going to sugarcoat it - three weeks isn't nearly enough time to get yourself ready to run a half marathon even for a light runner, much less a beginner.
I very much respect what you're doing for her, though. I wish you the best of luck. To help:
1. Despite what you may think, you need to alternate long and short distances. I wouldn't set a goal of finishing without stopping to walk, but you can probably get a decent amount of it run. I would try and run 4 this weekend, 6 the following weekend, and 8 over Columbus Day weekend. In between, shoot for 3-4 miles 3 times a week. Build as much endurance as you can, and enjoy the race without hurting yourself.
2. Good supportive shoes (don't add to the stress you're about to put on your body with Free Runs). Compression socks may help as well.
3. Stretching and body-weight squats will help keep legs limber.
Know you have #NTRunning behind you. Good luck, and, most importantly, congrats on the baby!