Its going to be a long road!

Hi Rory and welcome to BT.  There is a long learning curve to get the language and understand the gear and training prorams but it is all worth it.  Asking questions here and reading race reports is a good way to start your learning process.  I also recommend volunteering to help at any local races is a great way to learn and meet othe like-minded people which I found really sped up my learning process.

You could check out the mentor groups and maybe one of the beginner groups might suit you.  I'm in a mentor group for older athletes and there are several for beginners and you'll find everyone pretty accepting of your questions.  We all had to start somewhere.

I would just mention with regards to your knee - start out slowly when you've been given the "all clear".  You might even start out walking then progressing to fast walking, then walk/run (I started running 2 min, walking 2 min and building from there) and ultimately running.  I would also suggest that if running is a problem for your knee I have walked an entire 21 km in a HIM when I burnt out my knees on the bike and last August I walk/ran a 30 km for an ITU long course race.  No one would ever think less of you for walking - in fact they would cheer you across the finish line very loudly.

Make use of the training logs here (you can click on my profile pic or name and visit my profile or training log to get an idea of what you can do with it for yourself it really helps when you ask some questions for people to be able to see what your training looks like.

Whatever happens keep training and have fun.