If you were ever an endurance athlete (running, nordic skiing, cycling, for instance), you know what it’s like to develop a routine. That is, six days a week — for months at a time — of intervals, distance, tempos, strength, followed by a coach- or trainer-imposed (or self-imposed, if you were training alone and very disciplined) rest day.

The routine can be hell: the dread of the eight 400-meter intervals you’re going to have to run in just a couple of hours — just the thought of a negative split makes you nauseous; maybe it’s just tying the shoes and starting a 14-mile midweek run that’s hardest.

(The latter is my biggest hurdle, though once I’m out there, I’m good to go — no turning back, the open road in front of me, and the joy of being Alan Sillitoe’s lonely long-distance runner.)

Yet whether intervals, distance, tempo or strength, there’s always the post-workout euphoria that I anticipate.

What’s more, a mental clarity presents itself for the remainder of the day and even into the night, and I sleep like a baby after just about every workout-day. Sometimes the joy even arises mid-sprint, smack in the center of an interval day when I breathing and legs and mind all work together for a rare but treasured minute.

And I live for those moments.

Then there’s the seventh day: the day of rest; the off-day; la siesta — whatever you want to call it. (I just realized that whole seventh day/day of rest bit could be construed as very religious, but please know that this is not my intent — in fact, I’m a diehard atheist, but that’s a conversation for another day.)

At first, waking up without any regard for workout preparations is delightful. My mind is at ease for the next couple hours, too; I relish in the overindulgence of my breakfast — one I won’t have to worry about seeing in reverse during the run; I consider how good it will feel to lounge about, my only responsibility toward exercise to down a few liters of water by day’s end.

By the time lunch rolls around, though, I begin to feel somewhat purposeless. After all, there’s nothing like eating a meal after a huge workout has passed.

Even sitting down to do school work feels a little better knowing I’ve accomplished a tough physical activity; and if I sit down to work before an afternoon workout, I know my mind will receive a break when I lace up my sneakers.

Then my stomach contributes. I feel a little bloated from all the water, but the protein-heavy lunch on top of the pallet-pleasing breakfast is starting to hit my head, too.

With this, a little guilt comes in as the evil, exercise-oriented half of my conscience whispers in my ear: you know, you’re not going to burn this off today. It’s just going to sit. Unused. Wasted. Don’t you want to run it off?

Even though I know it’s my day off, at this point I’m driving myself crazy. I consider a strength workout, but decide it won’t be the same without the complements of cardio.

I begin to ponder a short, slow run because, let’s face it, I’m feeling pretty sedentary at this point. I know I need the rest day, but after eight hours, I don’t want to take any more time off.

So I give in: shorts, shoes, gloves, wind-breaker. I’m up and out, breaking my coach’s rule of an off-day. Four days later, mid-interval, my quads are screaming at me for not resting.

***

That was how it usually went during high school. I ran to train for Nordic skiing, and when skiing was over, I ran just to stay in shape and avoid the feeling of fatness that every off-day seemed to bring.

Without intervals to worry about during the summer months (my peers who wanted to break records in cross-country continued the coach’s routine throughout the summer, but I ran schedule-free), running without an off-day seemed like a no-brainer: no overly-taxing workouts didn’t seem to require any rest.

That was my attitude until the beginning of last week when, after three weeks of running with intense pain in both knees, I went to the athletic trainer for a diagnosis.

Here’s what she told me: loose cartilage surrounding one knee’s patella — which has led to an overuse injury in the form of a popliteal cyst behind said knee — and a strained MCL in the other.

Luckily for my runner’s self, neither is structural, and in four to six weeks I should be back to running. Having stuck to ellipticals (a nightmare for any runner, I think) and the stationary bike (not much better), the pain has even begun to subside.

But one word she uttered still hurts: overuse.

Should I have taken those rest days so many years ago? Should I have maybe considered six days of running with a day off, as my coaches had recommended, instead of my own 13-to-1 ratio?

Well, duh.

I wish I had listened to my coaches and my body and sucked up one day off each week.

Some might have said it was mentally strong to run without taking a day off, but I’d disagree from the seat of my exercise bike — or from any seat during the two days off each week the trainer [probably wisely] mandated — and say it was mentally weak.

Forget weak: it was stupid.

I was so worried about the state of my body and, at that, just feeling out-of-shape for one day that I couldn’t let it go. (With that in mind, perhaps it’s appropriate that this column is following the College’s “Love Your Body” week?)

Though I didn’t prior to injury, I can say now that I love (and miss) my knees. Before, though, I did them a horrible injustice by not providing the necessary rest. If my knees ever heal — knock on wood — I can say for sure that I’ll be resting up once a week, pampering my joints and enjoying a non-out-of-breath lifestyle for at least twenty-four hours at a time.

If you’re one of those exercise loons, give yourself some rest every so often — if not for your mind, at least for your body. It may not always seem like you need it, but believe me: you do.

Your muscles and bones will thank you whether you’re gritting through intervals, pumping up the final hill or hitting mile nine of your long, slow distance run tomorrow.

Rest up.