About you wrist... try to relax and don't over practice. People can last hours of practicing if they're relaxed (now theres a thought). Also bad posture at the start might be a bitch to fix up later. And to fuel your current "over enthusiasm" state, like noticing sez go find a good teacher at a local guitar shop or find a good books on basics. Lack of progress can really be a drag and discouraging.

Enjoy :)

And oh yeh, don't worry about big hands they can actually be good if you use them to your advantage. Later you'll realise having large hands will let you stretch out to other frets (this is a good thing by the way, people like me have to practice stretching to reach 5+ frets, not that you need to all the time but its always an option).