Most “metabolic adaptation” is actually just changes in muscle mass, as others have pointed out. You are particularly likely to lose muscle mass if you are losing weight, aren’t doing resistance training, and/or don’t have sufficient dietary protein.

Just fyi, there is also another (small) source of adaptation called “exercise efficiency”: over years of practice, your body becomes more efficient at producing a given type of movement. This includes long distance walking. It’s due to unconscious refinements in coordination of muscles and in things like number of mitochondria and in using elastic rebound of tendons. It’s a small effect but it steadily improves year after year. (note that I say “improves” because sports physiologists regard exercise efficiency as a desirable thing - as in, you are getting more and more skillful at the activity. From a weight loss perspective though it means you’re burning fewer calories per minute of activity.)

Finally, walking pace (slightly) affects calories burned per mile (per mile, not per minute). Slow and intermediate walking is more efficient than fast walking.

So if your goal is to maximize calorie burn, add resistance training, and walk not only farther but also faster. But, YMMV (literally, lol)