I went from exclusively cardio to weightlifting a couple years ago. I'm one of the "lucky" ones who can build muscle fairly quickly, so I loved seeing results after only two weeks. I did 5x5 for about 6 months and I felt terrific, but I got to be bigger than I was comfortable with. None of my clothes fit, I could only wear leggings and tunic style tops, which are not flattering on a muscular shape. Aesthetics aside, the biggest reason I stopped was because I was eating myself out of house and home. To maintain my physique I had to eat 2500-3000 calories a day, and that gets expensive. So, I stopped working out altogether for about a month, then I started couch to 5k. My cardio was terrible because I hadn't done any while lifting. Now, I run and do yoga a couple times a week, and I love it. My body is back to normal and I feel good. For reference, I'm 36, 5'7" 135 lbs pre lifting, 145 lbs at peak lifting, back down to 140 but I think my body recomped because my 135 clothes fit the same at 140. I've had two kids, and I credit lifting for strengthening my core to better than pre-babies.