The route itself measures just a hair under 100km with ~2700 ft climbing. With small incidental side trips along the way, most riders will put in a few extra km. (You can unwind with a few laps around the block if your odometer says you haven't done the full century yet.)
There are a good number of conveniently spaced cafes and stores along the way where you can get food and liquids, so if you don't feel like it, you don't need to pack much at all on this trip.
PLEASE NOTE: Because this route involves about 9 miles of gravel roads and paths, appropriate tires for that kind of surface are recommended. I've ridden those sections on a standard steel road bike with 700x28c tires pumped up to about 90psi and got through the dirt road sections fine (with some caution) but you have to pick your way through it carefully in some short sections. Anything but real hard, skinny racing tires should be be OK.
NEW FOR 2018: An alternate first leg will be used due to construction on the Road to Heart o' the Hills. It's a lot less climbing than the original first leg and cuts a few miles off as well. If you're not sure you can go the whole distance, there's plenty of "bailout" places along the route where you can just turn around and ride back to town.