The quick answer: Who cares!? For most of us it really doesn't matter how you measure and how precise you are. JUST GET OUT THERE AND RUN!
But let me digress and ramble a bit.
I began running in May 1994. Feeling happy and bubbly with 5 beers in my belly (and I did have a belly back then), the discussion with my fit coworker in her mid 50s turned towards her marathon training.
Lori (you know your real name) had (and has) a passion for running that kept her going, day in, day out, regardless of how she felt. I admired her for mental and physical toughness that got her out there training up to 60 miles a week.
I was 28 and had never run much, other than away from bees and dogs foaming at the mouth. But that beer was messing with my brain and in a sudden rush of drunken stupidity I told Lori that I would run the Los Angeles Marathon next year.
So starting the next day, I stuck with my commitment and got out there and ran. At first, only a block or two before my knees and ankles gave out. But I walked back home and tried it again the next day until I could run without stopping.
Long story short. I soon found myself not only running each night, but driving my car along the paths that I ran, jotting down the mileage to the nearest tenth of a mile. I had several pages of courses and their distance written down. Boy was I anal-retentive about it, to the point I would drive courses several times "just to make sure" they were correct.
But, at the time, I felt a need for precision because I was really getting into it and enjoyed tracking and analyzing my "statistics."
Here we are now in 2009, 15 years of running later, and my life and attitude have changed. I don't have the time, energy or desire to track my mileage down to the tenth of a mile. I still log my mileage, but generally round to the nearest half mile.
But it has been years and years since I drove a course just to measure the distance. I've developed a decent feel for the "pace" of a run, be it 6, 7 or 8 minutes per mile. So I simply measure the time run in minutes and quantify the mileage based on my estimated pace.
That said, there are some outstanding products out there now that can precisely measure your mileage for you. One of them is the Nike Plus gadget that syncs to an iPod Nano and lets you upload your mileage and pacing to a website. Click here for a decent write-up. I have a Nike Plus and think it is cool, but I prefer to just get out there and RUN. But if you're looking for data, this is a great, simple, useful and unobtrusive to use product!
I digress again. This talk about data and the Nike Plus is making me think about that cool NikePlus.com website again. Hmm.