Hi Cris!
The Boardwalk area resorts are my favorite! They are very close to Epcot and the starting line area is in the parking lot at Epcot. However, to get into Epcot from those resorts, you have to enter the International Gateway entrance at the back of Epcot and then walk to the front where the parking lot is. Since the race begins so early, you won't be able to walk through Epcot as it won't be open yet.
Theoretically, you could walk around the resorts, but that would be a very long walk and you don't want to do that! The Beach Club, Yacht Club, and Boardwalk are all host resorts for that race. This means you can take a Disney bus from those resorts to all race events. The bus will pick you up at the resort and drop you off right at the starting area. It's a great benefit of staying at one of the host resorts. You can look on the
Princess Half Marathon website, click on "Travel Details", and it will list all of the host resorts.
You could walk back through Epcot (assuming you have park admittance tickets) to get back to your resort after the race if you stay at a Boardwalk area resort. Or you can hop back on your bus!
I hope this helps! Please let me know if you have any other questions!