LONDON, England - The thought of the season-opening Australian Formula 1 GP helped Jenson Button push his body as he prepared for a new Formula 1 challenge as McLaren's leader on the track.Former team mate Lewis Hamilton has moved to Mercedes, rookie Sergio Perez has taken his place. Button has hopes of adding a second title to his name since winning the 2009 championship with Brawn GP.Now the most experienced driver on the grid, having made his debut in 2000, he's made sure he is fitter than ever as he seeks to win in Melbourne for the fourth time in five years.PHYSICAL AND MENTALButton said: "You do put in the extra bit of effort through the winter and because I've been training for so many years now, it does all add up. The number of hours I do now I wouldn't have been able to do four years ago - it would have been too demanding on my body. It's used to it now so I can put more time into it."But it's also about the mental training and putting yourself through all sorts."When Button joined McLaren at the end of 2009 there were many who said he had made the wrong move and was stepping into a "lions' den" where Hamilton was seen as the favourite. Over their three years together Button ended up out-scoring Hamilton. Though Hamilton was usually quicker, particularly in qualifying, Button was often smarter during the races.Rookie Perez needs to learn the ropes and show that he deserves to be in what should be one of the best cars on the 2013 grid.Button, whose smooth driving and intelligence have won new admirers since he joined McLaren, will be looking to lay down an early marker against a team mate who has said repeatedly that he is aiming for the championship right from the start. McLaren, which has always spoken of its drivers as equals, expect Perez to show that attitude from the first race - he is not being paid as a subordinate driver.MORE RESPONSIBILITYPerez said: "When you are racing for McLaren you are expected to win. Sure, there is the need to prove myself, but it is motivation rather than pressure that keeps me going. All the other guys in a top team have already proved themselves - I still have that ahead of me."Former McLaren driver John Watson, mindful of Hamilton's sensational 2007 debut when he ended the season second overall and ahead of double World champion team mate Fernando Alonso, said Button could have quite a challenge on his hands. As the senior driver, will have to carry more responsibility than he would have done when he had Lewis as a partner."I thought Lewis was going to be dominant and he wasn't. But that was Jenson joining Lewis. Now Jenson has Perez. It's never easy when you get a hot rod coming in, and Perez is a hot rod. How hot we don't know yet."NEW DESIGN, NEW WOESThe new McLaren MP4-28 was quick at times in testing but Button also felt the team was struggling to get the most out of what was a new design rather than a simple evolution. Button, who won the first and last races of 2012, was confident that McLaren would benefit over the course of the season."We wanted to build a car that would be strong throughout the year and that's why we've made so many changes to the way the car looks, the suspension geometry and how that affects the aerodynamics of the car among other things," he said. "It's a very long season and we want to be able to develop all the way through the year. Some teams might come with a car they used in Brazil, with a few tweaks, which means they'lll be strong at the first race but won't be able to develop like us through 2013."We want to be quick at the first race, we want to win from the word go. It's possibly going to be a tough race for everybody because it seems so close at the moment but we still go to Melbourne aiming for a victory."