Hike, cross train, hike some more, cross train some more.....
Swim, bike, hike, throw some light lifting in there, hike some more, you're good to go! At least I am, works for me.
And try not to be too overweight when you start a long hike. If you are way overweight, that's a tough start. 20 lbs or so, no worries, it will melt off.