One of the best way to build muscle fast is by climbing mountains fast.
It is always better to run against steep slopes, it helps burn faster and increase your metabolism.
Metabolism once increased helps in gaining good muscle mass. It is the single most important factor if you want to INCREASE body's cell production.
One good example of good health and stamina that results from running against the slopes is runners from Kenya. It is amazing how they can win so many medals easily in the Olympics.
Kenya has lot of areas covered from mountains and running is a routine activity followed by most people. Because people run against the slope their body adapts faster to run on normal plain track.
Most of these runners become skinny because they are not taking adequate rest! If you exercise and do not take rest, your body won't grow. Rest and feed yer body and see it growing. This might sound very simple and lame, but this really is very basic and most important factor to get good results.
So consider running on the mountains, it helps in maintaining good health and increasing weight as well.