I used to cycle alot... too many people getting run over these days, and the local DA likes to let the drunk ass murder drivers off...
Anyway read "the time crunched cyclist" by Chris Carmichael, it's old but still a great book for training for maximum FTP (Functional Threshold Power) and endurance without putting on 3-400 miles per week.
I take a little bit of exception to the above post:
If you can raise your FTP on a bike, you are going to get up a mountain faster than if you did nothing. Period, your body will be more efficient at converting Oxygen to energy and you will be able to hike longer, and at a higher intensity.
Now if you can make the same FTP gains doing a stair climber with a pack... you will get up the mountain faster than you would if you just trained on a bike. Thats the SAID principle.
If you make the same FTP gains hiking mountains with your pack. you will do the better than either of the previous examples.
That being said, the bike is a great way to learn how to feel your FTP, you can get all the gizmos in the world, heart rate monitors, power meters, blah blah blah, but learning how to feel your body and breathing and knowing "I can hold this exertion level for 20 minutes before I blow up" or "I'm at a pace that I can hold for multiple hours if I eat and hydrate properly" and also learning to eat and hydrate properly while in the middle of exertion, as well as what foods/ bars/ gels you can digest cleanly and which ones screw you up... thats also a great benefit of long road rides. the ability to experiment and compare how you did on a 20mile (or whatever distance you can do) ride today vs. a week ago or a month ago.