Preparing for a space shuttle flight or an EVA (spacewalk) is a very intense process. It's the physical training, of course, since going on a space walk is very physically demanding. There is also mental preparation and knowing your tasks. There’s knowing your equipment and how it works and how the gear might fail. Then there is the process of going through everything in your head, training runs - the things that you will be doing outside on a space walk. Going to Mount Everest is quite similar. You need to be getting your body ready, your gear, mentally preparing for the rigors of summit day and what leads up to it. It takes a lot of work.
I'd add that the team of guides and porters that took us through the Annapurna region last month was so hard-working and competent I began thinking they could do a Mars trek.