For this 21 miles roundtrip, 6000 ft-plus up and down hike, here’s how I trained:

• Running every second day for some months, usually in the 3-5 mile range, sometimes 7-8 miles.
• I worked that up to one (my first) half-marathon one weekend.
• On weekends, hiked up the local hills (~1000ft altitude change) at the highest speed I could muster.
• Also hiked a couple of 20 miles+ hikes for distance at a brisk pace. They didn’t have much in terms of elevation.
• For better measure, I did a night hike to see how to hike with a flash light and a head lamp. That was sort of fun, but not entirely necessary.
• Added: About 10 days before the hike, on three days, I also spent 30-60 min each on the stairmaster at the local gym.

Having done this, I had no issues with endurance up and down Mt Whitney. I did not feel like my legs were overexerted. It’s been two days since, and yes, I have some sore muscles in my calves, but it is entirely manageable.

One side effect: I feel like I’m in good shape — well, there’s also some evidence, see to the right :-) — and that feels good!