Can anyone explain why my brakes are weak?

I have a 1996 Toyota 4Runner. I noticed that after an hour's worth of driving on the highway, my brakes get really weak and soft. I have to apply way more pressure for the car to slow down, and I can't stop immediately, which frightens me. BUT, when I park the car for a couple hours, the brakes return back to normal! What is wrong with my car?