[MagicNinja]

if you do cardio you will burn energy; energy (calories) is used to either build muscle or fat (muscle if you are lifting; but if you eat HEAPS, not matter how much you lift it will start being stored as fat since your body can only build so much muscle at a time).

cardio will build a limited amount of muscle (but not in the same way that lifting will). it will also build cardiovascular strength (your heart will get stronger so you can run longer etc).

basically you will not be able to build much muscle unless you are giving your body slightly more food than you are using, since it takes energy to build muscle. you will lift slightly more as your body gets better at lifting weights even if you don't build much more muscle (the neurological pathways used to lift are strengthened i think). doing cardio basically means you will have to eat more food to build more muscle; that is why most people who workout for looks alone only do cardio in 'cutting' phases (where they want to shed weight while maintaining muscle mass). so if you are eating 2500 calories a day, your maintenance calories are like 2200 or something, that gives you 300 calories to build muscle. if you then go do 1000 calories of cardio you will now be losing fat instead of building muscle, as you are eating at a 700 calorie deficit. it is basically impossible to do both at the same time.

basically if you are trying to maintain your current weight while adding muscle, you should eat your maintenance amount of calories, then add on as many calories as you burn during cardio, then add on as many calories as you need to build muscle. if you want to shed your gut, keep doing what you are doing, then when it is gone, start eating more again so you can build muscle. but these should really be small changes.

often people just drink carb drinks to replenish the amount of calories they are burning during cardio (eg drinking a litre of gatorade while riding for an hour). i don't think too much cardio is bad for you, unless you were completely driven to bust yourself up (eg more than 5-6 hours a day).