My 3 cents worth....
You can usually tell when a player is out of shape when he's bending over holding his knees, sucking wind heavily and dragging up to the line of scrimmage. Last year in the 4th qtr. we had a lot of that. When you see a defense running step for step with the O in the 4th quarter, you know they're in condidion. Because our offense was able to move the ball and put points on the board, our D stayed off the field an appropriate length of time. Last year when we couldn't sustain drives, our D was right back on the field after 3 and out. We weren't in as good condition last year and so far, there seems to be a marked difference.