Welcome! I'm going in September for the half too!
I did exactly that last year - purchased the 10 day hopper and upgraded to an AP. It was a fair chunk extra. Like $200 or something. It's really only worth it if you're planning to use it for onsite hotel discounts, you're there for a couple of weeks, or you're planning to use it again. If you're planning to use it for onsite hotel discounts, you'll need to purchase one now instead of the 10 day pass so that you can access the discounts. Your pass registers the first time you use it. Because this obviously won't be until you're there, you'd need to call Disney hotel reservations and let them know your AP situation. Be aware that onsite hotel availability is EXTREMELY limited for that time already.
Also be aware that the Saturday and Sunday of half marathon weekend are block out days for the deluxe AP. There is a loophole that allows you to access the parks on a black out day if that is the day that you purchase/upgrade your pass. Last year I upgraded mine on the Saturday so I was able to go in after the 10k. We left on the Sunday for a road trip before returning to DLR later, so missing out on that day wasn't an issue. Obviously if you do this, you don't get the AP discounts until you upgrade.
Without knowing your circumstances (how long you'll be there for and whether you might have another trip planned within the year), I would tell you to really do your sums before deciding on an AP. It's a lot of money.
Just on accommodation, make sure you book yours ASAP as availability is already limited for lots of the close places.
