Cycling at any intensity will be much less effective for building aerobic capacity for long days in the mountains than foot borne activities. Cycling is very efficient and it also is done sitting down so you do not have to support your full body weight. Thus the transfer of training to your stated goal will be much less than it could be. Doing high intensity training on the bike will not help you in your long term goals either. For what you say you want to do cycling should make up on a tiny fraction of your overall training volume. I do agree that accumulating the needed volume at low intensity riding indoors will be mind numbing.