This particular passage in Enger's Peace Like a River is, indeed, an amazing description of something so mundane we rarely give it any thought. Breathing is one of those things we pay little attention to until something interrupts this rather involuntary body function. To write your assignment based on this model, you will need to think of something which you do with little thought or effort but something which you need to do in order to function in everyday life--or something you take for granted, whether you use it often or not. We can probably all relate to having a hangnail, for instance. We have skin around the edges of our fingernails, but we rarely give it much thought until we get a hangnail and it seems like everything we do involves that tiny little spot on one finger. Losing the full range of motion in a knee or shoulder joint works similarly to breathing; when it is all working well, we take it for granted. Not being able to bend an elbow or a wrist or an ankle might be interesting to think about, as well. Your writing model, of course, is Enger's passage; you will be able to write your own version once you select the activity.