This is going to sound a bit strange, but it really isn't that much more difficult to make the leap from 40 up to 100 miles. When I signed up for the H4H Dawn Raid last year, the furthest I'd ever ridden was around 40 miles.
Over the period of about 4 months, it was possible to add a bit more mileage each week, to get near that target figure. I specifically wanted that event to be the first time that i'd done a century, so I'd only been up to 90 miles before the day.
Once you break it down in to manageable sections, with decent rest stops and remembering to refuel and take on fluids, it was fine.
Also, picking the Help For Heros ride as the first one over that distance was also a conscious decision as the motivation of riding with the guys that have been helped by the charity is immense. It's bloody hard to say that your legs hurt, when the guy who's riding next to you tells you "at least you've got some!"
I managed to get out for another ride this morning - was touch and go in deciding whether or not to stay in bed at 5:30am, but once out there it was fantastic - saw deer, hares, kites, plus the usual pheasants, rabbits etc... and even a stoat or a weasel - not entirely sure which.
Even managed to pick a few daffs for the mrs, so earn a few points there as well