Kewell kicked off training with new club Melbourne Heart on Wednesday "” returning to Australia after he quit Melbourne Victory at the end of the 2011-2012 season to head back to England for family reasons.
The 34-year-old was unable to secure a deal in Europe throughout last season, and played just four games for Qatar club Al Gharafa before agreeing a return to the A-League.
But Kewell said his body is in good enough shape to deal with a long pre-season leading into the start of the competition in October, and is banking on that to give him the fitness he requires.
"I think I've come at the right time. It gives me the opportunity to work hard for three months solid before the season starts to get my body right for game one," Kewell said.
"Even when I wasn't playing, I was still working out six times a week. Obviously not to the intensity of a football match, but I kept on ticking myself over.
"If I get a good pre-season, stay fit, I can last up there with the best. The body's good."
Kewell is also hoping to win back his national team spot for next year's World Cup finals, but said playing well for the Heart was his top priority.
"To play good football for the Melbourne Heart, that's first and foremost. Anything beyond that is a bonus," Kewell said.
Kewell explained that his relationship with Heart coach John Aloisi - a former Socceroos team-mate - played a key role in his A-League return.
"We are good mates but we've got to kind of separate that now because he's my boss, I work for him now," he said.
"The enthusiasm he showed to bring me here ... nailed it straight away for me.
"As a player, you like to hear the kind of things that he said and about what he's trying to achieve. I want to be part of that."
"I know in myself that if I get a good pre-season, get fit, I know I can last up there with the best. That's the reason I've come out here now, to be prepared and be ready for game one."
While Kewell "” who will soon be joined in the Victorian capital by wife Sheree and their four children "” has initially joined on a one-year deal he suggested this season would not be his swansong.
"Considering I've had a bit of time out of the game I feel like I'm only 27 now anyway so I've got a long way to go," he said.
Aloisi said he was unsure of which position he would the man voted as Australia's greatest ever footballer.
"With the attack that we've got at the moment, with Golgol Mebrathu looking as good as ever "” he's got over a lot of his injury problems and is looking sharp, lightning quick as everyone knows "” David Williams, Mate Dugandzic, Ben Garrucio we've got some attacking power," Aloisi said.
"That's only going to help Harry "” and Harry will help them.
"I haven't decided which position exactly but there's a long pre-season to sort that out."
And the reward for Kewell after the three month slog will be a Round 1 meeting against Melbourne Victory in what is looming as a red-hot derby.
"I think it will be exciting. It's a game that sets the A-League alight, the Melbourne derby, and I for one will be really looking forward to it," he said.