I'm doing my first century in April...and I have GOT to get myself on my bike more than I am. I've done 80 miles at a time, but not the full 100.
Here's my plan for the next two months:
spinning (1 hour) 2x week
bike rides 3 times during the week for 1.5-2 hours each time out
weekend bike ride of 3 hours (increasing to 4 by the the beginning of April)
another weekend bike ride of one hour easy recovery
pilates 3x week and yoga 2x per week (I do pilates in the morning and yoga in the evenings on alternate days)
During the ride, I'm going to take it slow at first. I'll drink every 15 minutes and munch on something every 30 minutes (powerbar or banana). I'll stop at all the sags and eat and drink, but not for too long and not too much.
I will focus on time and not distance or speed. On my longer rides, it has helped to focus on the drink-every-15-minutes. If it's hot, I find myself drinking alot more than that.