We can't make time so we've got to make the best use of the available time we have. If you have an hour set aside for a regular run and you run the same route continuously you may find that after a few weeks you start to cover the same route faster as you get fitter. If you run for distance rather than time the session will end sooner and you won't gain the benefit of the minutes saved by covering the distance faster and the earned time is wasted.
My regular Saturday morning run is a hilly 60 minutes followed by a cool down walk to my house and as you'll see from the attached Garmin Connect links the distance I cover in the 60 minutes has started to increase as my fitness level has increased. If I was running it as an 8 mile loop the run would be finishing sooner and like I've said, I wouldn't get the benefit of the extra training minutes I've made!
If you compare the Heart Rate average and max for both runs it's evident that run 2 shows an improvement in my fitness level as the effort is less with a faster pace over the same route.
An hour is an hour and a mile is a mile but as you get fitter you can cover a mile in less time and get more training for your hour.