If the climb is going to take an hour, and you intend to get up it as fast as possible, you need to be able to ride at or just below your current limit for an hour. To do this, you will need to ride at your current limit for increasing durations and/or raise your limit by savaging yourself at an intensity above your current limit at shorter durations. It is best practice to do both. Add in some longer rides, i.e. your 100k road rides as a basic endurance ride and you should be good to go. Training for a hill like this is basically the same as training for a 25 mile time trial. The intensity will be comparable, but the speeds will be lower when going up hill.
If you don't care how fast you go, then riding your bike is all you need to do.