I have done several centuries, both as charity rides within a certain time limit before the course closed and on cross country day trips where we started at the crack of dawn and rode until we were a our desination, in one case 15 hours later. Most of the time my speed is about 14-15 miles per hour which should average to just under 7 hours, but that is without any breaks , stops, hills, headwinds, heat, cold, rain etc.
On the first century I ever did which was on the Mississippi Delta in 80+ temperatures, high humidity and with an intermitent 20 mph head wind, for the last 20 miles, I was off my bike for five minutes every 5 miles- just plain old butt rest time.
The best training for me is to first build up to longer periods/ mileage on a bike ie 15 miles without a break, 20 miles without a break, 25 miles a break. My comfortable maximum is 30 miles or 2 hours. Once I can go the distance, then I work on speed intervals over that distance.
Since most of what I do is cross country, endurance is probably more important to me than speed, but at the same time, speed is a factor, especially on rides where the course closes at such and such an hour. If this is the case, quite often they will have a cut off point or time and unless I can make that easily, I won't try for a century. Still it is time to race the cutoff times as well.
The group (womantours.com)that organized the last cross country I did sent aloong a 6 week training agenda which had a good variety of short speed rides, and endurance rides so that by the last week we were up to 6 1/2 hours in the saddle.
I would think best use of your time would be several shorter more intense rides, using intervals to build strength and speed and one or two longer rides for TITs (time in the saddle) time.
Whichever way you go, take comfort in the fact that if you can do 75 miles comfortably, you can probably push yourself to do 100 or more. Have fun and remember to take a day off the bike in between sessions.
marni