The best way to look at protein isn’t as an aid for muscle growth, but as an aid for muscle adaptation. If you’re training like a bodybuilder, you’re telling the muscles to adapt like a bodybuilder’s—e.g., to maximize hypertrophy. If you’re building a base for an expedition or thru hike, on the other hand, you’re not going to end up Arnolded out, you’re going to start your trip as a stronger hiker.
Protein will cause some muscular growth as a single type of adaptation, but only in the muscles that are carrying you and your pack around, and only to the extent you use them. Protein will also encourage new red blood cell formation (to combat against hemolysis, or the breakdown of red blood cells common in athletes who put their feet through the paces), new mitochondria formation (to improve energy metabolism), and new immune cells (to ward against upper respiratory infections, a common ailment linked to aerobic exercise).
Regardless of whether or not you consider yourself an athlete, you need an athlete’s dose of protein. It’s not that you won’t adapt without it, you just won’t adapt as well or as fast. You’re limiting your potential for improvement (within a given period of time). If you’re going to put the time into adequately preparing in the first place, you might as well get everything you can from it!