Whether you keep track of your running by miles run or time run doesn’t matter as long as you have a system you use consistently that works for you.
Most people keep track by counting miles. That’s true even for people who keep track of individual runs by minutes (e.g., “I ran for 42 minutes today, so it’s going in the log as 5 miles”). Even though it has no intrinsic meaning to the human body, a mile is our standard unit of measurement, so we might as well go with it. (An American friend who lived in Europe for a couple of years started tracking his running in kilometers and jokingly noted how much more fun it was to say, “I’ve been hitting 150 a week” than, “I’ve been doing 90 a week.”)