Does it matter if I run 3 miles instead of 3.1 miles? Only if you’re running a 5k. All right, so if one tenth of a mile doesn’t make a difference does a quarter mile? How about a half? I suppose it depends upon how far you are running and why you have stopped.
If you are supposed to run a mile and a half, then yes a quarter mile matters. If you are supposed to run twenty miles, stopping at 19.75 probably isn’t a big deal. If you are cutting a half mile off a run, you ought to have a good reason. The “I didn’t map the distance properly,” or “I don’t have a Garmin,” are not going to work.
Some runners will run back and forth in parking lots or up and down their street to get that last little bit to hit their mile goal. If it’s under a quarter mile, I don’t worry about it. I will do at least another mile during the day walking the dogs, and I’m sure I will walk at least a quarter mile around my house. You don’t need to be a mile Nazi.
If you are cutting quarters or halves off of every run, that’s a problem. If you are calling home for a ride at mile five of a ten mile run, you may need to re-evaluate your goals or get a running partner who won’t let you quit. “I just wasn’t feeling it,” doesn’t work with me because if you don’t get out there you’re never going to “feel it.”
Injury is totally different. If you develop a sharp pain at mile five of a ten mile run, that doesn’t recede quickly, you should probably phone a friend to pick you up or limp back along the shortest route possible. If you fall and it’s more than a scrape, I understand cutting it short. If you’re vomiting, sure go home and go to bed, but if you feel better after emptying your stomach, keep going.
If you are training for a marathon and consistently cut your runs short, you are cheating only yourself, unless you are convincing your training partner to do the same. Then you are both screwed.
There is value in struggling through hard miles. There is value in pushing yourself past the farthest distance you have ever run. Those achievements will lend you their strength during your future hard runs. And there will be hard runs to come especially if you want to improve.
Here’s the thing with running, No one can run your miles for you. No one can do the hard work but you. Running keeps us honest, because if you don’t run the miles, it shows.
Running doesn’t tolerate cheaters or liars. If you don’t run the miles, your day of reckoning will come. And you will beg for mercy.
ultrarunningmom 
So true! I’m someone who runs up and down the street to get to my exact mileage. My neighbors think I’m nuts. I also must always end my run on an even number, because I’m slightly crazy 🙂 But I agree with you, that really a few tenths of a mile doesn’t matter. However, cutting short your runs by a half mile or more is only going to hurt your performance on race day.
Those last two paragraphs should be printed on a t-shirt!