Terry arrived from Chelsea last summer and has been a driving force in third-placed Villa's promotion bid.
Speaking ahead of today's QPR clash, Bruce said: "He still trains and looks after himself.
“That's the reason why he is playing at the level he is.
“John doesn't drink any more, he has his own dietitian. He still has that pride.
"Physically, the Championship is equivalent to the Premier League. Years ago there was a gulf.
"When I first started in management it was huge but now there is no real difference.
"Our team cover as much ground as a Premier League team. Physically you have to be ready for it and there are more games.
"If you're not ready for it the Championship can be brutal experience.
“Am I confident John can maintain it until the end of the season? Oh yes."