Why is long term consistency so important in this sport? Running 50 or 100 miles is not a matter of precision and fine tuning. It is a matter of deep rooted mental and physical strength and endurance which is developed through a series of micro adaptations that we make over the course of years. No matter how hard you train in the 2 or 3 months leading up to your first 50 or 100 mile race you are going to get worked over really hard in that race. Of course there are random exceptions, but everyone I know has been physically hammered by their first ultra. On the flip side of this I see folks (myself included) who have trained and raced consistently for a few years (or in some cases for decades) who have been able to make these gradual adaptations such that they can race every few weeks and only the occasional "race gone bad" has the extreme physical effect that those first few ultras seem to have on everyone. I remember shortly after I ran my first 50 miler someone told me that if I kept doing them fairly regularly that my body would "learn" to do this without even being sore the next day. At the time I thought there was no chance of this. Now, 5 years and about 35 races later, I rarely have very much soreness after a 50 mile race.
Anyhow, if you've read this far you might now be thinking, "okay, good point about long term consistency, but this doesn't make short term consistency unimportant." To some degree I think it does though, because in my experience the most likely way to be consistent over the long term is to not over do short term consistency which in almost all cases seems to lead to eventual injury, fatigue, or burnout that limits long term consistency. This is to say that rather than focusing too much on trying to run a certain amount everyday, or a certain amount of time/mileage each week I think most ultrarunners could benefit a lot from just adopting a lifestyle of going out and running when their bodies and minds (and logistics of day to day life) allow for it and not so much when they don't. Running every single day for a year or running 100 miles a week for a year (if you are one of the rare few to pull this off without getting injured or burnt out) isn't going to make you nearly as capable of an ultrarunner as running a consistent and challenging amount of mileage/hours per year for several years. A few weeks without much running, or even a month, will do virtually nothing to set us back once we have built up all these micro adaptations that this kind of long term consistency leads to. But we can only get to this point if we can stay generally healthy for a long period of time.
How do we best do this? By not focusing too much on short term consistency and just taking individual days as they come and letting our bodies dictate when and how much we run. Of course this is just my opinion about all of this. And of course there are many folks who defy this theory, but I would argue that most ultrarunners would actually run more (and faster) over the course of the long haul if they focused less on trying to run a certain amount each day or week and just ran each day and each week what felt right, taking into account the physical, the mental, and the logistics of day to day life as it comes at us. When your body and mind feels good and you have the time in your day to go out and run like crazy then go out and run like crazy. But when you're not feeling good or you just don't have the time to squeeze in a run without it being too much of an extraction on the rest of your life, then just do what you need to do to take care of your body and/or your life and don't stress about not running enough that day or that week. By not running on these days you'll actually be making yourself a better runner over the long haul.