To be in prime condition, train daily through the year. This helps you space your regimen without having to worry about it at the nth hour before a departure.
Maintain a 60:40 split between cardio and weight training in the gym. Building stamina is of prime importance. While walking the mountain trail some journeys like the Everest Base Camp takes 14 days (both ways) to complete. And there are longer journeys as well. Light weight training is also important. It helps strengthen the body overall. The arms, shoulders and the legs absorb the maximum strain as you trudge up and down the steep gradients with a backpack/ day bag (if you are lucky) in tow. Body strength is always useful.
Alternate 3 days each between cardio & flexibility training and resistance (weight) training respectively. Indulge in any sport of your choice, cycle and run on treadmill or outdoor inclines. Remember to play the sport for 60 minutes, cycle for 30 minutes and run for 30 minutes. This helps keep the legs active, maintains stamina through the year and ensures a good cardio regimen. Keeping the body joints flexible is also important. Working the upper back, lower back, chest, biceps, triceps, leg curls and abs are some of the key exercises in the weight training. Spend 60 minutes in the gym on weight training days. It’s important to keep switching the regimen to avoid monotony.
When the body is fit you can actually feel it responding on a trek and/ or a river run. The mind beams confidence, the body feels light and you are able to cover ground much faster than the others. It’s a superb condition to enjoy and revel in; wonderful overall. (This was best demonstrated on the Everest Base Camp Trek. Keeping pace with the Sherpa guides is always difficult and no easy task. Walking alongside them is even better. But being appreciated by them is many notches higher.)