I'd tend to agree with some of the other posters that a 5-hour century is not really realistic -- that's an average of 20 mph. Honestly, I would make the goal to finish -- it is definitely tougher than 65 miles, especially when that's the longest you've ridden, and by the sound of it, only once or twice last year.
Since you've only got about a month or so, here are some general suggestions for you:
(1) Keep doing your spinning classes and rollers as much as you can during the week, and if you can extend the sessions from say 1 hour to 1 1/2-2 hours that would be beneficial. Don't overdo the intensity though or you will be burned out and fatigued for your endurance rides which are more important at this point. If you can get outside during the week, go ahead and do it.
(2) On the weekends, you have to get in some saddle time whether it's on an indoor trainer or outside -- work your way up each weekend (try for a Saturday long endurance ride -- nice steady pace, where you're not breathing heavy, and you could carry on a conversation with someone while riding). This is just a rough guide, but try and get in at least a 40-miler this weekend, the following weekend 45, the next one 50 and the weekend prior to the century 55 or 60. If 40 is easy for you this weekend, then extend it 5 or 10 and also increase each weekend accordingly -- ideally I would try to shoot for at least a 60-65-miler the weekend prior to your century ride, but whatever you can do will be okay. If you're stuck on an indoor trainer, then try for a couple of hours or longer of a nice steady pace. If you can't last that long indoors, maybe try for 2 or more sessions in the same day just to break up the monotony.
(3) On Sundays, try to follow up your Saturday long ride with another pace ride that is also on the long side -- say 60%-75% of the distance you did on the Saturday, so your weekend might look like this -- Saturday 50 miles, Sunday 30 miles -- this way you're at least accumulating as much saddle time as possible while allowing for some recovery between rides -- again, make sure you're riding at a nice easy/steady pace -- the goal is to finish and not be totally wasted.
(4) While you're doing these weekend rides, make sure you also work on your eating/drinking -- get used to your sports drink, gels, other food (fig newtons, powerbar, clif bar, etc.) and plenty of it -- stop at convenience stores if you need to refill or get food, or whatever. It's easier on century day since there will be rest stops stocked with what you need.
(5) The week of the century, just do some very easy riding of no more than 1 hour or so, say on Tuesday and Thursday, and maybe the day before the century, and start to do some carbo-loading and drinking lots of water to make sure you're hydrated and ready to go.
Hope this helps. Others may have some better ideas, but based on what I've done, this should at least give you a fighting chance.
Good luck and keep us posted of your progess and the outcome.