Indeed. I've been doing long runs for my cardio. I average running 10 miles a day, 4=5 times a week. What i do is start out slow, maybe an 11 minute mile pace for a few miles and then go faster as I get going, I'm a slow starter for some reason. By my 3rd to last mile I'm going 9 minute pace, then 8.5, the 8. The last half mile I just give it all I have got left in the tank.
My endurance has improved in a very short time. In the winter I was only able to go about 5-6 miles, and went at a 10 minute pace all the way through.
My longest run is 15 miles and every one in a while I only push out 7 miles or so. It depends on how long I have for my workout. But I ALWAYS start slow, and end strong.