A couple quotes from Hadfield's Reddit account:

We are trained to be able to do everything onboard - we have to be. It takes many years.

To be an astronaut who orbits earth and stays for a while, it needs to become your main life's work. It takes: physical fitness to the highest standard, an advanced technical degree, and a proven ability to make good decisions when consequences matter. Then apply to the Space Agency of your country, and compete with the thousands who also want to fly in space.

Sources:

http://www.reddit.com/r/IAmA/comments/18pik4/i_am_astronaut_...

http://www.reddit.com/r/IAmA/comments/18pik4/i_am_astronaut_...

> It takes many years.

One hell of an understatement: Hadfield was accepted as a trainee astronaut in 1992 and first flew in 2001. For the ISS alone, he got his slot in 2010 and trained until December 2012.

I met a backup astronaut (well, one who hadn't yet gone up to space) a few years ago...among her amazing engineering achievements, which I thought would've been enough for going into space, she was also a Division I NCAA track athlete.

The trials for Navy SEALS and other elite military ops are well-discussed...I wonder if NASA has any extreme tests they run...such as dropping off an astronaut on a remote island with nothing but flint and tinder and a shaving mirror and having to build a shelter with functioning refrigeration by the end of the week.

You should read Mike Mullane's "Riding Rockets". It has some weird details about the application process, including strange medical procedures...

There's something along those lines in Rocket Girls, whose author generally plays close attention to realism. (No idea if that specific part is "true" though)

Does spending hundreds of hours making fine movements underwater in a pressurized suit count?

There was a recent car commercial, where they rapidly flashed scenes of the process of car design, with a voice-over reading "Bring out the designers. Bring out the mechanics. Bring out the nerds." Each sentence would show a stereotypical example of each. The designers were in a bright Apple-store-like office, wearing fashionable glasses. The mechanic was a long-haired guy holding a wrench.

And the nerds were astronauts, in launch suits, presumably walking towards a space launch.

Dodge Dart commercial - http://www.youtube.com/watch?v=lUxPYXvTElA

It wasn't quite so idealist back in the shuttle days. Yeah, the pilot and commander and mission specialists were definitely amazingly qualified, but the payload specialists were pretty normal people.