If the inquirer is a fitness oriented type who sees me at the gym frequently, he may be the sort of person who assumes that I follow a periodic routine, and that I run pretty much the same amount every time I go out. The most frequent expectation is that I run so much per day. My usual quick reply is that most runners track it by weeks and months, not by days. The "per" designation is just a handle by means of which one may discuss averages, which themselves may or may not be meaningful.
My own routine is surely not much different from most thoughtful runners. I record all pedestrian miles covered while wearing running clothes for the purpose of "working out" on a daily basis, often in miles to two decimal places when running a known measured route, whether I'm going for a short walk or a long run or some combination thereof. A training session for me can be anything from one easy mile to over forty. The distance almost always predetermined, usually at least days and sometimes weeks in advance.
In turn, those daily accumulations add up to seven-day weeks. In my log I record not only the week as measured from Sunday to Saturday, but each day I calculate a new sliding total of the last seven days, including on rest days. That number can be a good predictor of how well I might expect to do the next time out.
Weeks add up to months. For a while I also calculated the past month's sliding total as 30 days or 31 days, until it dawned on me that a month varies in numbers of days, and activity tends to cycle on a weekly basis, which throws variables into the number. Therefore I changed sometime last year to adding total mileage for the past 28 days on a daily basis regardless of the number of days in the current month —g four weeks, which is constant, and makes more sense as a guide to "how I'm doing" as far as recent mileage. So while my last 28 days might say, for instance, 180 miles, the month total of 30 or 31 days may wind up as 200 or more. That's okay.
Week and month totals tend to build and diminish over the course of a year, depending on complex variables including weather, upcoming races, and explicit training goals. I'll usually set a goal mileage for a month on the first of each month, using those factors, along with recent experience as a guide, and then will rough out the weeks, particularly the long runs so as to hit somewhere near that goal.
Keeping track of detailed training data is valuable, more so than having some vague notion about how much I'm running, which would likely result in not running enough to reach ultrasized goals. More data is better than less data in a sport where a scientific approach can give a person with minimal intrinsic skills (me) a greater advantage than a casual approach. So that's how I measure it.
As noted, other people will ask about my running, and it helps to have a way to discuss it. Because I train an unusual amount on an indoor track at a gym, there are quite a few people who see me on a regular basis, and know that I sometimes run fairly long mileage there. It's not unusual for someone to come up and ask: "How much do you run every day?" There is no answer to that question if taken literally that is both easy and correct. I could do a mental calculation in the knowledge that this past week I ran 48 miles, so could say "about seven", when the real answer is that I did a couple of short runs, a 10-miler, took a couple of days off, and ran 25 on Saturday. Or I could say "about 40-50 miles a week", or I could say so many miles per month. But when you're flying by someone who asks the question in passing, that's not the time to get into a big discussion or to present a lecture on how runners train.
In January I passed on the track an older gentleman I've never stopped to talk to who asked me, "How did you do on your last marathon?" The last race I'd run was Across the Years. I don't know how he knew that I had been in a race recently. I responded quickly: "Which one?" He scrambled for an answer: "Any one." I said "A hundred and forty-two miles!" as I glided out of conversation range, knowing he wouldn't have a clue what I was referring to. He shouted back, "Very good!" in a voice that clearly indicated he was confused and didn't understand my answer.
Still, I'm always happy to discuss this topic in any amount of detail with persons who are really interested and have the time. It makes little difference how one describes his routine. It is what it is.
A rose is a rose is a rose. — Gertrude Stein