Muscle Gain

My workout regiment has always been cardio. I could run for hours, and actually, I have run for hours in marathons. I figure if it is not broke, why try to fix it? People tell me that I should cross train but I have really no desire to. Besides, if I find a workout routine that works for me, why would I jeopardize it with trying to throw in a modification that I may not like. So I have started a Stemulite supplement program to see if that would help me with a muscle gain instead of trying to do it through weights.