I was 68 when I biked the CF. The last 200 km are not flat. I have no experience with the other routes. There are a couple things to think about in regards to days with real hills. You can always send your luggage (painniers) ahead to the next place you will sleep. It if is very hilly, then send it a short distance. Take extra time in the hilly sections, that is don't try to go as far in a day. Take lots of breaks, and do some walking your bike up the hill.
The most important thing, from my perspective is to make sure that the bike has been adjusted to your (and your dad's body. I had a professional bike trainer set up my personal bike to the exact dimensions for my body at home while training for some 200+ mile 2-day rides. That included changing some of the handlebar components, modifying my clipless shoes so that my foot was at a different angle to help improve knee function, and adjusting my high-end special bike seat to the right angle and height.
I rented a bike in Europe and I brought my own helmet, bike pedals, bike shoes, and seat with me. Prior to removing the seat I took detailed measurements from of the seat angle, the distance of the top of the seat to the bottom of the pedal stroke, the distance from the front of the seat to the handlebars. The distance from the handlebars to the bottom of the pedal stroke. This allowed me to triangulate all the critical measurements in adjusting the bike to my body. Then on the rental bike, I installed my seat that my body had trained with, my pedals that my bike shoes were set up for. Then I adjusted all the rental bike dimensions so they closely matched the bike I trained on.
Assuming your dad has trained on a bike and it has been adjusted to his body, I really wouldn't worry to much about the flattest route, just be flexible in sending luggage ahead and/or making hilly days, short distance days.
Good luck.
