I do as many hill climbs as I can to train for backpacking over passes. Works very well. I tend to stay in the saddle to work my quads more, stand up and push smaller gears at a high cadence to build wind. My knees after a lifetime of construction work, especially lots of tile and wood floors, can't take running. Biking is the best thing I've found for getting in shape for the mountains. Higher cadences- at least 70 rpm, are less strain on your joints than pushing 100" gears. Don't be afraid to change your position on the seat, and alter the attack of your legs. A few degrees will change the muscles worked and prevent fatigue.
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Charlie