It's good for a big middle aged man of that size, no doubt. Remember, he was a world-class athlete back in the day, and he is still maintaining his fitness. Also, quarterbacks tend to be very good all around athletes, often with good endurance/strength, and although not lean, certainly not bulky. As an example, the starting QB on my hs team was also a pretty big guy, probably about 6-0, 185lb, started on the basketball team in the Winter, then came out for track in the Spring and ran the 1600 in around 4:12. This guy was a great athlete, as was T.A..
What's my point? I don't know...