Whats the minimum amount of cardio to build up your heart and lungs

[buffgator]

I have started doing cardio because i have spent my whole life bulking up and ive never done anything to take care of my heart and lungs. I feel lethargic and wiped out all the time and I get winded very easy....as in walking up 2 flights of stairs. I have been doing 20min of cardio a day 5 days a week at a heart rate of 120-140. I am 6'2" 17%bf and 310lbs. I dont care if it leans me up or not I just want to feel healthier and have more endurance.

[38jumper38]

try some cardio early in the morning, and after work out, go slow every day and make some progress daily, after a while you will be running all you want.