Hi Michelle!
Thank you for bringing your question to the Disney Parks Moms Panel!
The Glass Slipper Challenge is a very popular race. It will generally sell out within two hours, if not before. The 5K lasts a bit longer, but not much because of more limited spots. My advice is to know exactly when it opens, be logged into your Active.com account, get on the site about ten minutes prior to opening and refresh until it's available. You should have no trouble if you do all that!
One of my favorite things about runDisney events is that they are perfect for both runners and walkers. RunDisney's official trainer is Jeff Galloway. You can find
training programs for each race and challenge on the runDisney website. His training is geared toward running, but he has made the run/walk famous for runDisney enthusiasts. I would take his training program distances and just walk them instead of running or running/walking.
You will need to maintain an overall 16 minute-mile pace for the race, so I recommend training at a 15 minute-mile pace or lower. This gives you a little flexibility to have fun on the course and snap some great photos! The 16-minute mile pacers will begin after the very last runner starts, so starting close to the front of your corral will also give you a little extra time.
I hope this helps! Please let me know if I can help you in any other way to get ready for Princess Half Marathon 2017.