YOU HAVE TO CONSULT A NUTRIONIST.
You've got a few things that prevent you from gaining weight:
1) A fast metabolism
. The only way to get around this burn-everything-you-eat metabolism is to eat more than your system has the capacity to burn.
2) Higher percentage endurance muscle fiber vs. strength muscle fiber. There are two big types of muscle fibers. One is really good at aerobic exercise
. The other is strength fiber. Increasing body mass requires you to avoid endurance training as much as possible, especially during the first three weeks. You must lift weights to increase your proportion of strength muscle fiber. Heavy weights.
A) Eat about 4000 calories a day.
B) You'll need to stop any endurance training to gain weight. Aerobic (endurance) exercise simply isn't compatible for your goals. Your body will spend it's energy growing endurance muscle (i.e. wiry, skinny muscle) if you do low impact, long term exercises. You can continue doing a little aerobic exercise (10-15 minutes 3 times a week) but during the first three weeks it would be best if you did none.