Being physically fit is one of the most important aspects for success on a high altitude trek. As trekking in Bhutan is challenging, you must train intensively before coming for a trek. Your training must include cardiovascular endurance (via aerobic training), strength endurance (through strength conditioning), hiking-specific training (via hiking with a pack) and altitude training (via ascending to required height). During your training, you should be planning to progressively ramp up your speed, duration (time or mileage), and pack weight of weekly training hikes to give you hiking-specific conditioning that cannot be matched by any other sort of training.
Include running, walking on an inclined treadmill, doing stair stepping or stepmill training, trail running, working on an elliptical machine and walking up and down hills. While bicycling, rowing and swimming are also cardiovascular options.
Most important in strength training is to be sure you maintain proper form at all times in order to prevent injury or strain. Training with free weights, bands, bodyweight exercises, or gym machines will help you build overall strength in the upper part of your body and legs. Developing strength in your upper back and shoulders will help you while carrying a pack and using trekking poles effectively. While descending and ascending, calves, hips, quads, hamstrings and muscles are all involved and hence strength endurance is required in all areas of the legs and hips.
For hiking conditioning, hike on trails having an ascent and descent, gradually increasing your backpack weight with each outing until you are at your target trekking pack weight. You must try carrying an average pack of 4-5 kg over a distance of 5 km at first season and you then gradually increase till you can carry 5-6 kg for 7-9 km.
As most of our treks go above 11,000 feet you will also have to do some altitude training. Medical history suggests that people who have had ascended to a minimum height of 3000 meters 30 days prior to start the trek has less chance of suffering from altitude sickness.
One useful high altitude training technique is to include interval training in your weekly program. Climb steadily on a steep hill or sets of stairs for several minutes pushing as hard as you can while you go up and then recover coming down. Repeat it for anywhere from 30-45 minutes. For hill walks, add weight to your backpack on a regular basis until you can carry 5-6 kg the whole time.
Although fitness is important, people should not worry much about altitude. Most of our trekkers are above 50 years who could complete our treks. There is no evidence that altitude is likely to bring on heart diseases, which is previously been not diagnosed. If you can exercise and train maximum at home, you should not have an increased risk while trekking. However if you already have a known heart disease and your exercise is already limited by symptoms at your place, you must consult your doctor before coming for a trek.