Wayne Rooney has admitted that he once turned up seven pounds overweight to Manchester United's pre-season training. The 26-year-old striker also cast doubt on his long-term career by admitting he struggles "to walk for the first half an hour" following a game.

In extracts in the Mirror from his book, My Decade in the Premier League by Wayne Rooney, the England forward writes about the summer of 2009: "Early July: the first day back at pre-season training. I'm like most blokes, I put on a few pounds after a holiday. Even if I don't train for a week, I put on two or three, but when I get back to Carrington for the first day of work, I'm in for a shock. The scales in the club gym tell me I've put on a few more pounds than expected – seven. Seven!

"Then I remember – I drank a few bevvies while I was away. I'm stocky. I'm not like Ryan Giggs, all bone and lean muscle. But I gain weight quite easily. It's not a problem though. It's not as if the manager is leaning over my shoulder as the numbers come in, tutting and making jokes about me eating too many chip butties. Besides, I know I can shift it in a week or two. All the players are given loose training programmes to stick to while they're away, but they're optional.

"The club like us not to go overboard on the eating and drinking in the close-season break, so if I go abroad I like to get into the hotel gym three times a week to work on the treadmill and do some weight work.

"That way I can be sharp when we get back to training and the running will feel easier when the pre-season games start. But coming back for pre-season after a few bevvies and a few weeks away from a ball is physically tough."

Rooney then hints that he may struggle to play on in his current role as United's focal point for many more years. "As a striker I need to work hard all the time. I need to be sharp, which means my fitness has to be right to play well. If it isn't, it shows," he says. "It would probably be different if I were a full-back. I could hide a bit, make fewer runs into the opposition half and get away with it. But as a centre-forward for Manchester United, there's no place to hide. I've got to work as hard as I can, otherwise the manager will haul me off the pitch or drop me for the next game.

"There's no room for failure or second best at this club. When it comes to nutrition, all the players know what to eat and what not to eat all year round, but we allow ourselves some luxuries. During the season, I don't think there's any harm having a take-away now and then. The club always has someone on hand to talk to me about diet if I need them.

"Physically I've taken a bit of a battering over the years; being lumped by Transformer-sized centre-backs or having my muscles smashed by falls, shoulder barges and last-ditch tackles, day in, day out, has left me a bit bruised. When I get up in the morning after a game, I struggle to walk for the first half an hour. I ache a bit. It wasn't like that when I was a lad. I remember sometimes when I finished training or playing with Everton and United, I'd want to play some more. But football has had a massive impact on my body because my game is based on speed, power and intensity.

"Like any player I'm fearful of getting a career-ending injury. I could be in the best form of my life and then one day a bad tackle might finish my time in the sport. It's over then. But that's the risk I take as a player in every match. I know football is such a short career that one day, at any age, the game could be snatched from me unexpectedly. So I want to decide when I leave football, not a physio, or an opponent's boot.

"But the fear of injury or failure has never got into my head when I've been playing. I've never frozen on the football pitch. I've always wanted to express myself, I've always wanted to try things. I've never gone into a game worrying."

The striker, who was dropped from Sir Alex Ferguson's starting XI for last Saturday's 3-2 over Fulham at Old Trafford then taken off on a stretcher with an injury which will sideline him for four weeks, also discusses the previous time he was left out for United, following a 5-0 win over Wigan Athletic on Boxing Day last year.

"I'm happy at Manchester United, despite the downs that sometimes take place at a football club. Like when we stuff Wigan 5-0 on Boxing Day. I go out for dinner with a few of the lads, and our other halves, to a hotel. The next day, the manager pulls me up and tells me he's not happy and doesn't feel I've trained properly. He fines me, but there's worse to come. I'm dropped for the next game, on New Year's Eve, against Blackburn. At a lot of clubs, people wouldn't bat an eyelid at players having a night out six days before a game. But that's the difference at Manchester United and a mark of the high standards the manager demands. It's a big deal, another lesson learned.

"The following week I had to sit in the stands and watch us lose 3-2 to Blackburn. It's the worst feeling when that happens. It was terrible. Blackburn looked certainties for relegation, yet we were worse than them. As I watched I feel desperate and helpless, just like all the other United fans watching the defeat unfold."

Rooney suffered a gashed leg after coming on as a replacement against Fulham and he admits any absence through injury "messes" with his head. "When I'm injured I get wound up and nervy watching games. It's like being a fan all over again, probably more nerve-racking than actually playing. It's so frustrating. I can't influence the game at all. I'm helpless. There's nothing I can do to change the result and help my mates win. I try to keep a happy face on when I'm around the other lads afterwards, but it's hard.

"When I'm out injured I know I can't train or help the lads prepare for the next match so, typically, I get grumpy, a bit like someone would when they have to give up smoking or coffee, I'd imagine. I have breakfast with the team in the canteen and when it's time to start work, they go one way to the training pitches and I go the other to the physio's room and the gym. They're playing small-sided games out on the training pitches and I'm getting checked out. It's boring.

"I'm a fidgety patient. I get snappy. I go quiet. I don't get fed up with the treatment or the physios and club doctors, I just want to get out there and play in the practice games like everyone else. The worst thing is that the rehab process messes around with my head. I feel left out at the club. I miss the banter and the crack in the dressing-room. As I'm not fit enough to play, I don't even get to spend the night in the team hotel with the rest of the lads before the next game. I have to stay at home, then drive into the training ground the following morning for some more boring recovery work.

"Still, it could be worse. Owen Hargreaves had operations on both knees and didn't play for 18 months. He was a regular for England and United before his injuries. He missed out on so much, I don't know how he got through it. A couple of days is bad enough for me. I reckon my head would go if I couldn't play football for a year-and-a-half. Players become a spare part when they're seriously injured. They become forgotten men around the club."