well, you've got lots of lead time, that's for sure. if the weather is good start riding now and try to get your mileage up. not sure what your fitness level is but you might have to start small? 5 miles, then 10, then 15, etc? basically a century ride is about being active all-day - that takes some getting used to. if you can't ride all day right now you might try doing other things combined with cycling over the course of 8 hours.