Villa were relegated from the Premier League in 2016, losing their top-flight status for the first time in 29 years.
They came fourth in the Sky Bet Championship this season and Bruce, who was appointed in October 2016 with Villa 19th, knows it has been a long slog.
He said: "Eighteen or 19 months ago this club was in a mess but the experienced players have done the job for us and we're delighted about that.
"As a one-off it's fantastic if you go up through the play-offs, although I would love to have been second and sitting on a beach now.
"It was always going to be tough but what has got us through is our defence over two games.
"If any team deserved to win the game tonight I think it was us."
Mile Jedinak's first-leg header ultimately settled the tie on Saturday as the sides slugged out a scrappy stalemate at Villa Park.
Stewart Downing hit the bar for Middlesbrough late on but they did not register a shot on target.
Villa came close through Lewis Grabban and Conor Hourihane and were comfortable against a poor Boro side.
Boro spent around £50million in the summer to chase an instant return to the Premier League after last year's relegation, but came up short.
Boss Tony Pulis, who replaced Garry Monk in December, was upset Villa goalkeeper Sam Johnstone was not sent off by referee Mike Dean after he handled outside the area with three minutes left.
He said: "I think it's a disappointing decision from one of our top officials.
"It irks me a little bit but the teams who finished third and fourth are going to Wembley and that's justification of what they've done over the season. We've just fallen short.
"In both games you need things to go for you and tonight there are one or two decisions which have not gone for us.
"I have a fabulous chairman, everyone wants to do things for you at the club. We have to improve next year."