So I was choosing between Roopkund and Sandakphu. Roopkund is at 16,400 ft and Sandakphu is at 11,899 ft. The altitude is more important than the distance and altitude is what makes trekking difficult. You can walk 20 kilometres in Mumbai easily in around 5 hours if you walk at 4 km/hr, I generally walk at 6 km/hr. However, on the mountain, your speed decreases as the oxygen reduces and you tend to get tired faster. Hence Roopkund is more difficult even though the distance is less.Problems with chasing altitudeThese days trekkers tend to chase altitude and this is disastrous. As the height increases, the oxygen decreases and you put yourself at the risk of AMS (Accute Mountain Sickness, a person suffering from AMS is descended from the mountain immediately, so if you chase altitude and get AMS midway, your trek is over!). It is always a good idea to start trekking at low altitudes and to let the body get used to a high altitude trek for the future. If you can breathe at 7,000 ft once, your body will take lesser time to acclimatise there later. Hence, try to increase the altitude slowly to prevent AMS.