1:21 PM ET

Al Horford was a free agent once. He didn't want to do it again.

Horford agreed to a two-year, $20 million extension to stay with the Celtics.

"For me, I really wanted to be a part of what we have going here," Horford said after the Celtics held a shootaround at their practice facility. The group started to make strides in the right direction and I felt like I was coming back here again.

There is a real purpose to trying to win and the group that we have is a really good group. We know we have a lot of work to do. It is difficult to find this type of stability.

Horford had a hard time learning that. Horford decided to join the Philadelphia 76ers on a four-year, $109 million contract after three seasons with the Boston Celtics. Horford was dumped by the Philadelphia 76ers after a disastrous first season in which he proved to be an awkward fit with star big men.

He was traded to the Celtics for Kemba Walker in his first move as president of basketball operations.

Horford said he took less to remain in Boston than he could have gotten as an unrestricted free agent because of the experience and the joy he felt after returning to the Celtics.

Horford said you want to be in a place where you have a chance to contend. It's more than basketball for me. It's something that I'm really excited about because I feel a real connection with the people in Boston and the Celtics. It's something that's special, and I'm really grateful for the opportunity, because I got that opportunity and Brad believed in me and brought me here.

Horford will be 39 years old when he is signed. Horford said in his 16th NBA season that he wasn't sure if he would live to see another one. He attributes his improved long-range shooting to his time with the Atlanta Hawks, as well as his time with coaches Mike Budenholzer and Kenny Atkinson.

As the Celtics await the return of defensive anchor Robert Williams III from knee surgery, Horford has held down the inside fort.

Horford wants to clear up uncertainty about his future and show his commitment to the Celtics, who enter Friday with a league-leading 18-4 record.

When asked if extension talks had begun, Horford said, "I think this was something that was going to happen at some point or another." There were a lot of things that the team had to deal with. I wanted to make sure that this didn't drag out until free agency and the summer and all those things and just once the time was right, to focus on the season.