how does the body adapt?

I would say a little bit of both mental and body adaptation especially if its something your body is not accustomed to doing on a daily basis. I remember when I began doing suicide drills for football they were initially hard in the beginning, but over time I started saying to myself let's go you can do this and eventually as I continue to drill it became easier so I assume my body adapted over time with a little push from the mind.

I definitely believe it does. Muscle memory is a proven phenomenon and I personally believe (no medical proof) that this is related to adaptation. As the muscles get used to doing a certain task, they get trained to do it in the most efficient way possible. I agree with Rtossas that some of it is mental too. Mentally you become accustom to the strain/pain and it has less of an effect in the long run.

Where I've really noticed what I believe to be adaptation is in my lifts. If I do the same isolation exercise for months on end, after a while they feel like they lose their effectiveness. I can lift a ton but I'm not feeling the same "burn" as I do earlier on. Now if I do a different isolation exercise, still focusing on the same muscle group but in a different way, I not only get the burn back but usually will have a bit of DOMS to go with it. For that reason the last couple months I've started really mixing up my lifting program. I've tried to find as many different exercises as I can for each muscle group and I mix them up for each workout.

So far I've been happy with the results. Overall my muscles feel and look like they're gaining more mass where I had previously plateaued. Additionally, I'm noticing more significant strength gains. Tony Horton talks about muscle confusion in his workouts and I while I don't follow his plans, I do think there is some validity to the concept.

I think you guys are missing the point. The OP wasn't asking about muscle adaptation making things easier, but rather speculating on how he's become so accustomed to a specific type of workout (i.e. mountain biking for 45 minutes) and going past it is extremely difficult.

THAT barrier is what I think is mostly mental.

If it's a question of if your body adapts to strain to perform more efficiently/effectively, it definitely does. I don't think there's any room for debate on that one.

You're right, I misread what Jackson was getting at here. Even then I think it's a mix of mental and physical. If you do the same workout for the same length of time every day for a long period, I absolutely believe your body adapts to that and pushing beyond will become tough. Our bodies run on cycles and when your workout becomes routine and predictable, I personally think it then becomes a part of your body's cycle as well.

I've got no medical evidence of this but looking at how our bodies seem to adapt to handle any repetitive stress I have to believe that it can certainly adapt in this way as well.