Hi,
you need to get close to what you will be doing in the event. If it's 60 miles or so, then you need to add 10 miles a week. If it is a multi-day event, you need to do training rides back to back as well.

If the course is in the mountains, you need to train there. Do one really hard ride a week. Then do a couple easy rides (basically M-W or T-Th) to keep things going.