If you aren't fit and you've got dodgy knees, I'd give the hundred a miss. It's a helluva long way if you're struggling.
If you insist on doing it, then set your computer to the "average speed" screen, and don't let that go over, say, 14mph. Stop every 10 miles and actually get off your bike, stretch, eat, drink, stretch again, then head off for another 10. Just break it down into a series of short rides rather than one long one.