Hey there and welcome to the TR community!
I think either of those plans would be a good choice for the event you’ve described. One might be preferable to the other, though, depending on how you plan to approach the race.
The Gran Fondo Plan generally features steady, sustained efforts at about Threshold or VO2 Max. If you want to ride up the climbs in your event at a steady pace without worrying about the need to surge too much, then the Gran Fondo Plan would be the plan to go with.
Example workouts:
The Climbing Road Race Plan is pretty similar, but its workouts often feature high-intensity bursts of power. The workouts will have you going hard above Threshold, then settling back into a rhythm at about Threshold once again. These surges are meant to simulate attacking/responding to attacks going up climbs, so if that’s how you want to ride your race, then the Climbing RR Plan would be the better choice.
Example workouts:
As for volume, we might recommend starting with a Mid Volume Plan if it’s your first time doing structured training. Intervals can be quite demanding at times, and if you combine that intensity with more volume than you might be able to handle right now, it can lead to excess fatigue and burnout. You can always build up your volume over time!
This article will give you some good additional info on that:
Hope this helps – feel free to let us know if you have any other questions!