Originally Posted by Doctor Morbius
I think it would depend on what body type you inherited from your parents. Are you an Ectomorph, Mesomorph or Endomorph? Most people don't fall neatly into these categories, however. I'm an Endo-Mesomorph and gained 22 Lbs while lowering my % bodyfat in the past year while cycling 8 - 12 hours per week at around 75% MaxHR. So it is possible to gain weight and burn fat while cycling.
From your description - 5'8" and 130 Lbs - I would classify you as an Ectomorph. Ectomorphs typically have a hard time gaining weight, especially with endurance activities. However, you seem to be able to do so. Don't be afraid to consume an extra 1,000 to 2,000 extra calories per day (something I can't do!) and go to the gym and lift. Concentrate on the larger muscle groups as building up one's biceps won't really pack on the weight.
Yes, I would say I'm one of the few that fits squarely in the ectomorph category as I'm extremely slender and have a very small bone structure. I've always had a hard time gaining weight except when I exercise quite a bit. I think the reason for that is not just building muscle mass, but the simple fact that it stimulates my metabolism and I end up eating about twice as much as when I don't exercise or do anything physical. So far I've seen a few decent suggestions... eat more protein, more calories, and more carbs. Steak, chicken and fish are higher in protein from what I've been told, however they seem to have zip for carbs. Potatoes, bread and pasta are the only things that come to mind as being high carb foods. Any other suggestions? Unfortunately I'm an incredibly picky eater and there are a LOT of things I don't/won't eat and am not willing to try.