I think the correct question should be stated as if i’m training for a (insert race distance) how many miles a week should I run. However, if you’re not training for a race, the answer could be different.  Right?

Here’s how I see it.  In terms of getting the right amount of exercise for our bodies, various studies have revealed various results.  Bottom line is we need to move everyday.  This could be running, biking, walking, standing, playing with your kids, or just about anything that gets the heart rate elevated.

I would see nothing wrong with answering the latter of the two questions by saying you should be active for one hour every day.  This could be running one hour a day, if you like to run or enjoy running.  It doesn’t mean running as hard as you can for one hour every day.

If you’re training for a specific event, then the answer becomes a bit difficult to answer and training programs obviously become involved.  My answer for a marathon would be, as many miles a week you can run without getting injured and still enjoying it.  This could also hold true for a 5k!!

If you’re comfortably running 50 miles a week, and you get enough sleep and rest, then adding a few more a week could improve your overall marathon time.  Some runners have improved their marathon time by doing nothing else but adding more miles a week.  Just be careful! Don’t run too hard or increase by more than 10 percent a week as you could increase the risk of injury!

I once asked a colleague who has coached many runners to collegiate and Olympic glory what he considered the proper mileage totals to succeed in distances from the 5K to the marathon. “That’s easy,” he said. “You want to run as few miles as you can and still win.”

Source: How Many Miles a Week Should I Run? | Runner’s World