| If your main goal is cardiovascular endurance and you continually find trouble with running long distances there are other options. Swimming and biking are two other activities that may suit your body type better. Between an 8 and 9 minute for 4 miles is not bad at all.
My opinion, keep running, learn to pace yourself better (bring a stop watch if not on treadmill), and do not get discouraged with your current pace. Gains are meant to be achieved over time, not over night. | 
