Hi Laura! I think you will love the Princess Half Marathon. It is a really fun race!
You are correct that Disney transportation buses are only provided to and from the host resorts for each race weekend. Since you want to stay at Saratoga for your DVC points, you can simply arrange for a taxi to take you. (assuming you don't have your own vehicle) You will want to arrange it in advance at your front desk. Also, be sure to get a copy of the road closures and alternate routes that your front desk can provide for you.
I have both taken a cab, and driven myself to races at Disney. While it's not as convenient as hopping on the bus, sometimes it just works that way, and I have never had any trouble. Be sure to leave at least 2 1/2- 3 hours before the start of the race to allow plenty of time. If you leave that early, you will also avoid the long traffic lines.
You can find all of the travel information on the
brand new rundisney website.
Please let me know if you have any other questions!