Minutes or miles, its a question that comes up either at the beginning or end of your running career. If you just getting started, you might wonder if it's more important to put in the distance or the time.
As someone that has done both, the answer to this lies in the goals you are setting.
If you new to running, does it help you stay with running to have a fix distance or time? I believe that your should start with time. This way, 20 minutes is 20 minutes. How important when you are first starting is it to complete 2 miles (assuming your run a miles in 10 minutes on average)? It is probably more important for you to complete 20 minutes than it is to complete the 2 miles. Why? Because consistency is what you want when you are first getting started. Whether you are running 3 or 7 days a week, you want to be consistent in making sure you reach a base that you want to achieve.
Before I reached the age of 40, I was a mileage maniac. If it said 5, 6, 7 or whatever in my training program, I did it, regardless of time. After 40, I realized that it took me longer to complete the prescribed distance. Frustrated, I found myself changing plans half-way to the next race goal. Along with that change, the expectation of time goal for the next race dropped. I felt like I wasn't in good enough shape to make that goal (since I was already dropping to lower mileage).
For those of us that look at running at being a lifetime 'sport' or exercise that we wish to keep for life, running for a set time is more practical than a prescribed distance.