playtheday, well done on your first run. Please don't be disheartened, it just takes time. I would echo what the others have said about a running/walking combination. You can do whatever's comfortable, run 1 min walk 1 min, or run 1, walk 3, whatever feels ok and then slowly start building up the time that you are running and reduce the walking time. It won't be long till you are running more than walking and then running all the time. Also, definitely slooooooow down. You might feel that you are almost going to be walking, but this is fine. It will improve your fitness and give you confidence. As to breathing, try and relax. Pull your shoulders up to your ears and then drop them (you might realise you were holding them up a bit so try to keep them down). Also try smiling (easier said than done sometimes!), it really does help you relax which should help the breathing. Don't worry too much about holding a conversation, I struggled with that for ages! I think nowadays it is generally considered a good idea to stretch only after a run, not before. A brisk walk is a good warm up. And well done again, you're on your way!
DrNorth, nearly throwing up at Army Fitness, getting covered in mud, buying trainers. You are such a running geek! Huzzah! 
Paws, well done on your 20. Don't worry that it was hard, you did it! Can't belive you are going out tonight, much respect!
Well done Doyouthink too! Another 20 miler. Awesome.
Moshi, when you say "enjoy" a 2 week taper, do you mean indulging in bootcamp and other intense sessions?? 
Had lovely walk in woods this afternoon and looking forward to 6 miler tomorrow, if only to work off all the pannetone 