That isn't really a fair example Hoove. If someone is fat and out of shape, than any athletic action is going to make them better. It is a question of comparison. Running long distance versus sitting on the couch all day: which would make a better athlete? Running long distance, of course. But if you were to place the overweight kid into a program of sprints, olympic/ explosive weight lifting, and plyometrics, he would excel more in baseball than if he were to run long distances.
Athletes in general are healthier than non-athletes. Doesn't matter if you are fast or slow twitch muscle in that regard. Tiring less quickly really doesn't apply to baseball. Long distance trains your muscles to go for long periods of time with less than maximal effort. Baseball is played in short spurts of maximum effort. It is a different animal entirely.
You can do what you want. I've already said that numerous times. Do what makes you happy. Life is too short to dedicate every breath you take to getting better at baseball. If cross country makes you feel good, more power to you. However, the question was asked about slow twitch muscle and the effects it has. I wanted to post the information here so people could make their own decisions.
Well, what are your goals? Weight lifting is actually a smaller part of the puzzle than many realize. The saying "muscle is made in the kitchen" is very true. You can workout til you are blue in the face, but the body needs excess calories in order to build bigger muscle. Weight gain is based on caloric surplus or defecit. You need to find your Basal Metabolic Rate (BMR). That is the amount of calories you burn just by being alive. If you add to that the factor of your activity (which would be very high because of X-country). That number would be your daily calories used. So, if you take the amount of calories that you eat in one day, and then subtract the total burned, would have your caloric surplus or defecit.
I'll use myself as an example. My BMR is 2063 based on height, weight, and age. From that I would add about 900 calories for daily activities like workouts or just walking, etc. So daily my body would burn around 2900 calories. I eat 2200 calories a day, so 2200-2900= -700. I have a 700 calorie a day defecit. If you take that for the whole week, that would be 4900 calories a week, which is roughly 1.5 pounds lost (3500 calories is roughly one pound).
So if you want to gain weight, you need a surplus. Aim for a certain amount to gain every week. Start with one pound a week and see if you can keep up with that. Keep track of your food and then eat more if need be. There are many "hard-gainers" like you that have really high metabolisms, so you might have to eat 4000 calories a day in order to gain.