Haven't ticked Cook itself but have climbed my share of +3000m peaks in NZ.
My advise is its mind over matter. The stronger mentally you are the more likely you are to succeed.
Mental strength and motivation in the mountains come from having:
-Fitness, the fitter you are the harder you are to kill essentially. You can't be too fit.
-Skills, you need to have a solid base of technical skills and be able move fast and safely over a variety of terrain.
Nothing like knowing you are as prepared as you can for the route ahead.
Id suggest doing a course with AGL or such and then attempting a few smaller peaks to build a base of experience before giving Cook a crack.
Otherwise hire a guide to take you up if you have the fitness but lack the experience.
As other have said, the mountains take no prisoners. Don't be scared to pull pin and come back another day.
Good luck, you'll be buzzing for weeks if you pull it off!
Bookmarks