The England striker is confident of having a big impact at Old Trafford this season.
"I'm 30, and I don't want to wish my career away.
"In an ideal world I'd like to play out my years here. I had great moments last season - a hat-trick in the Champions League and a goal in the Carling Cup Final, but the derby goal was fantastic.
"It was one of the biggest, most pure adrenaline rushes of my career. It's alongside that Germany hat-trick, scoring twice in the FA Cup Final, and that goal in St Etienne.
"I was fit for 43 games. The pity is I missed the ones at the end when we were playing for things. The amount of injuries I have had is exaggerated.
"It can be annoying, but you learn to live with it."