Time out. You say your legs give out before your lungs... but this does not necessarily mean that your muscles are weak. My legs would give out if I were carrying 400lbs on my back too - and that is what you are doing, because most obese people have less muscle than non-obese people. Your body size is still the major obstacle here. The heart is not built to pump enough blood around your body size under stress such as exercise. If you can lose some weight, you will increase your exercise endurance and add about 20 years to your life span...