London 2012 two-year countdown excites Olympic stars
Hoy does first lap of 2012 velodrome
Olympic champion Sir Chris Hoy said the 2012 Games have taken a step closer to reality after he became the first person to ride in the London velodrome.
The four-time cycling gold medallist was taking part in an event to mark two years to go until the Games begins.
"I had goose bumps on my arm," he said. "To me now the Games are coming alive."
Michael Johnson was among the sporting stars participating in the countdown event, the American running on a temporary track in the Olympic Stadium.
Retired British basketball player John Amaechi, who never got to play for Great Britain at an Olympics but did come out of retirement to help England win Commonwealth Games bronze in 2006, shot hoops in the basketball arena.
Johnson beaten by schoolgirl at Olympic Stadium
Hoy, who was knighted after winning sprint, team sprint and keirin gold medals in Beijing two years ago - to add to the 1km time trial title he won in Athens in 2004 - rode in the velodrome which he helped design.
"When you walk in, you can really envisage it on race day," he told the BBC.
"To see pictures of it and to see stuff online doesn't do it justice, when you get in there and see it in the flesh, it's amazing.
"You can't stop thinking about the crowds cheering for Team GB in here in two years time, it's important that you do because it gives you that little carrot to chase.
"Every training session you are giving 100% but on the hard, long road rides in the rain in the middle of winter you can allow yourself these daydreams of what it might be like, and now I have got a visualisation of the arena.
"They have done an amazing job and the track is not even in yet. I really believe it will be the best velodrome in the world.
There is so much history behind the Olympics and people in London will get an opportunity to go to an event and say they have taken part in it. They will have that memory for the rest of their lives
"It's so exciting to be here and to me now, you can actually get a sense of what it is going to be like.
"There is a real buzz about the place, that's two years to go, what's it going to be like next year?"
Hoy was joined by London Mayor Boris Johnson in taking a bike around the velodrome infield.
"The bike was a little bit big for him but he was very keen to get on there, so fair play," added Hoy. "He got a bigger cheer than I did from the crowd which I was very disappointed about! You have got to play second fiddle sometimes.
"He is a great guy to have supporting the sport. It's great to have him on board."
US track legend Johnson ran with a group of schoolchildren in a short race started by London 2012 chief Lord Coe.
"It's great to be in the stadium and to envision what is going to be taking place in a couple of years," said Johnson, who won gold medals at three consecutive Olympics, including the 200m and 400m double at the Atlanta Olympics in 1996.
"The excitement is building and it's great for these kids taking part today," he added on the BBC.
"It's a once-in-a-lifetime experience. I had that opportunity in 1996 when Atlanta hosted the Games. It was great for the home crowd to be able to come in and experience it.
"There is so much history behind the Olympics and people in London will get an opportunity to go to an event and say they have taken part in it. They will have that memory for the rest of their lives."
Lord Coe used the milestone to urge fans to consider what they want to get out of the Games.
"There are two years to go until the Games begin and we want everyone to start planning their once in a lifetime experience in 2012," he said.
"We are well on track - the venues are nearing completion and our plans are coming together. Now it's the public's turn. How will you spend the summer of 2012?
Amaechi shoots hoops at basketball arena
"Whether it's buying a ticket, becoming a volunteer, being part of our education or culture programmes - there are hundreds of ways you can get involved. Don't wait - to ensure you don't miss out, start planning your Games now."
Mayor Johnson, meanwhile, launched an appeal for 70,000 people to volunteer to work as everything from city guides to press officers at the Games.
More than one million people are expected to register their interest in the London Volunteer programme to become a 'London 2012 Games Maker'.
And Coe said the volunteers are the difference between "a good and a great Games".
He explained on BBC One Breakfast: "Volunteers are the people that make the Games. I have seen the Games through a number of prisms.
"We want people to get on the website but I would say don't make a decision quickly because it is tough.
"It will be three weeks of shift work. You may be in the stadium but you may not be seeing the 100m. You may be working downstairs, making sure the athletes get on the track on time.
"There is a whole mountain of jobs that the volunteers do. Without them we don't have a Games."
Beijing medallists feel weight of responsibility
London's Trafalgar Square is holding a day-long event with interactive games for the public.
Tickets for the Olympics do not go on sale until the spring of 2011 but 1.4m people have already registered their details.
"We have got 75% of the tickets going through public ballot which is the biggest proportion than any other single sporting event in history," added Coe.
"It's really important that we get people that want to be in the venue, in the venue and particularly young people because we want to inspire another generation of people into sport."
London beat Paris to be named as the host city of the 2012 Games in 2005, but since then the budget has spiralled to £9.3bn - nearly four times the estimate that helped win the bid - amid the global financial crisis.
A contingency fund makes up £2.2bn of that and it has already been dipped into, to the tune of £95m in October 2008, to fund the building of the 3,000-home Olympic Village.
However, work on the construction of the Village, the stadium and other venues remains ahead of schedule, as Hoy, Johnson and Amaechi will demonstrate on Tuesday.
"I am really pleased with how things are going," said Coe.
"Here we have a stadium that is structurally pretty complete. The seats are going in and it will be finished by next year which gives us a chance to start fitting it out and turning it into a stadium. This is fantastic progress."
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