Hey, bimmer4me - you can't dream of being a sub 4-minute mile runner yet run those long distances for 6 days a week.
Sadly to say, it also depends on your muscle type - I used to do track back in the days - there are fast-twitch muscle types and slow-twitch muscle types. People with fast-twitch are incredible sprinters while the other make very good cross-country competitors. There are some people in a 'gray area' between these two but not many. If you want to be a 4-minute mile runner, you mainly sprint-train. You also do tons of weight lifting with your legs. Tons. A 4-minute mile (1600m) is pretty much a sprint all the way.
Regardless of whether you want to do cross-country style (long long distances) or sprints, both are healthy methods for cardio workouts.
@ Bavarian - one of the easiest changes you can do to alleviate pain to both your knees and your back is to make sure you are running properly. You have to have good form and good footing. You mention impact against your knees and maybe your back as a problem - perhaps you can solve that not only by getting the best shoes but to change the terrain you run on. Runners have grass, dirt, gravel, sand, rubber, mud, you name it. All vary especially in softness and grip. I find treadmills to be quite funky as they have a peculiar bounce to them - it takes a while to get used to them if you are running on them after running for a while on other terrain - you may have to optimize your shoes.
Running on concrete/blacktop is usually the most punishing to your knees, ankles, and back - you can feel the solidness of the ground reverbrate through your whole body each time you step down. Running on dirt/grass is probably the easiest for people to handle as dirt/grass is pretty soft, lacks bounce (unlike treadmills) and is slightly more slippery allowing your feet/legs to move a little more freely. Since you will lose grip, your mile time on dirt/grass most likely will be a little slower than your mile time on a rubber track field.
I'm sorry about my outlandish behavior - there's too much blood in my alcohol system.