On the other hand, habit is the runner's worst enemy. Most of the adult runners that I work with in training share a problem that I have -- which is that we get stuck in certain habits as a runner. These habits can be mental habits: such as setting too ambitious race or training goals, getting too excited when workouts go well (or too upset when they go poorly), reducing all the various purposes of training to "mileage," or obsessing too much about the details of training. The habits are also physical: we return to the same workouts, month after month, year after year, forgetful of the fact that teaching the body to grow in new ways means that we must train not just harder, but differently!
Running well requires a routine. As James reminds us--as the Stoics did long before him--the most necessary thing in life is to develop good unconscious habits so that we don't waste mental energy or willpower on basic things. I firmly believe that the most important workout we do is the simple easy run that we do almost unconsciously every day. That run establishes the routine that makes you into a runner.
But within that routine, we need variation. Not everything can be unconscious. The way to reach your potential as a runner is to think as clearly as you can about your own strengths and weaknesses as a runner and then work specifically and purposefully over a period of months to address those strengths and weaknesses. Yes, this means changing our habits and making them better. Those could be mental habits (like goal setting, controlling your reaction to workouts, racing more confidently, or believing that you can race well without training at the absolute limit) or they could be physiological habits (like developing speed, form issues, handling a certain mileage level without breaking down, etc.)
The deep philosophical point that James is driving at is that we are creatures of habit. Which means that we are always in danger of becoming mere automatons, driven by our habits. The only way to resist that fate is to create the habit of refreshing, renewing, and critically examining our habits. We need to be creatures of good habits. This is especially the case for older, adult runners who have been at this gig for a while.
Routine is not a bad thing in itself, just as habits are not always bad. Willpower is in short supply; it has its limits, which is why we need habit. The essential thing in intelligent training is to set up a habit of reflecting on your own habits as a runner. Else, you risk getting caught up in the cycle of suffering.
Establishing good habits as runners doesn't mean we won't hit injuries or setbacks or just simply get tired of running and move onto different things. Intelligence has its limits, and sometimes we don't even know what habits we need to improve. The most important habit of all may be the habit of remembering that we run for enjoyment -- which seems to me to be something entirely unlike "habit" or "intelligence" or "will power" or "purpose." So, if you are someone who thinks about your habits all the time, may this post cause you to examine the value of that very habit!