I don't think you're obligated to use the credit by the end of the year; if you just go to a ticket office/airport and have them reissue the credit, you'll be granted another year's extension.
On the other hand, it's a good reason to travel. Depending on your mileage balance (or lack thereof), and being located in So. Cali., hop on a flight to OGG or KOA!!! Loads are lower in the fall, prices are cheaper in the islands...a great time to go.