I use to be someone who was very fit cardio wise. I was awesome at the 100m, 200m, 400m, 800m and cross country, but then high school stuff kind of got in the way and I progressively stopped doing sports. I can't even run 100m without being puffed out. Luckily for me I have a very fast metabolism and all the junk I've eaten over the years haven't added to my weight (I fluctuate between 55 - 58 kg @ 5'8 in height)

What kind of cardio can I do without losing weight/muscle? I'm doing weights about 4 times a week btw.