Am I fit enough to participate?

Probably. You need to be able to cycle continuously for one hour at an average speed of between 13-15 mph to be comfortable on a section of the ride. You will then need to be able to get on your bike again do another hour at the same speed after a 5-10 minute break. You don't have to do a whole stage if you don't want to.

I would suggest that you build up to cycling for a full hour (if you can't already do so) and then concentrate on getting your speed up if you need to. Once you have achieved that then try two consecutive hours and do two or three of these before the ride itself. Buy a bike computer - these tell you how you are doing and are great motivators.