I may have posted this previously, but at any rate, here's my approach, which has worked successfully for the Whitney Main Trail day hike (2004, age 59), a Mt. Charleston (Nevada) day hike (2009, age 64), and Grand Canyon rim to rim day hike (2012, age 67).

I start the training about half a year before the big event.

My usual exercise regimen is to walk 2 miles a day except for Saturdays and Sundays. I also add a one mile jog on Tuesday and Thursday and do a 2 mile jog on Sundays. I keep this schedule while I'm doing my event training, except that the 2 mile Sunday jog is replaced with the following.

The training is a 3 way approach, vacillating from one week to the next. One week is a 22 mile hike -- nothing necessarily out in the country, pre-planned (and measured) hikes on city streets. (When I START the training I first do an 8 mile hike, then a 15 mile hike, then its 22 mile hikes from there on out.) Week two is stair climbing -- I use the 17 steps in my 2 story condo stairwell. The drill is to go up and down them 150 times. (When I START the training the first time is only 100 times but from there on out it's always 150 times.) The third exercise is to run 3 miles, except for about mid-way through the training I do a 5 mile run and do another one in one of the last weeks.

All the above is "kicked up a notch" with about 3 months left in the training by doing everything wearing a 22 pound backpack. And I wear it for my normal daily walks and jogs too.)

I find that the 3 exercises are a good mix and that the training is absolutely necessary. If not to actually be able to do whatever event your planning then to at least make it much more pleasurable.

For a Whitney event you have to add in your acclimatization approach as well. When I did my Whitney day hike I flew from home in lowland Florida and spent a little time in LA before driving up to the Sierra. I actually traveled from sea level (cruise to Anacapa Island) to Mammoth Lakes, where I stayed for three nights. After the third night I drove south to Lone Pine. I walked up to Lone Pine Lake that day. I stayed at the Dow Villa in Lone Pine. The next day I drove up to the portal and just hung around there all day, staying at the Dow Villa again that night. The next day was my hike. I found that drinking lots -- and often -- helped with the altitude.

Hope this approach gives some good ideas . . .