Muscle strength vs. heart/lung strength This question may be very basic, so please have patience with me. 

When I run for an hour and try to cover as much distance I can within that hour, the thing that's limiting my speed is my heart rate going up and feeling out of breath. My legs don't feel all that tired afterwards (or even during the run).  I think it is because I do a lot of non-running exercise that improves strength, but not so much endurance. (Weight training, cycling, circuit training, etc.)  So my legs are strong but apparently my heart and/or lungs are not. 

How can I train to increase my speed at mid-distance running (say 6-10 miles) when muscle strength is not the limiting factor but heart rate/oxygen capacity/I-don't-know-what-to-call-it is?

