I'm a naturally thin guy, so i have to really limit my cardio in order to keep my weight up and add muscle mass.

So I only run once a week. Lately I have been running stairs at the beach. I get to scope out all the bikini wearing hotties, and get a workout at the same time.

By the last leg I'm usually barely able to keep from passing out or throwing up, and my quads will be sore for a few days. But it doesnt make me shed pounds like "jogging" does, and it's just enough cardio to keep my 5K run times around 21mins.

If you're at the beach, why not go swimming?
If the surf's rough, then go to the pool.
Good low impact cardio workout, I think.

If you want to jog then do so. However I would implement a jeg workout at the gym. Also if you are having trouble keeping weight, I heve read in several sources that guys like you(cant gain much weight) need more carbs than the ave. joe does to get the same effect. I am just the opposite. I will gain mussle like it is nothing, but I put on a pound of fat or two for each pound of muscle.