1) Firstly to be a professional biker type you should probably be in a reasonably sound shape first. You know, maybe walk the dog once in a while or if you don't have one, race your grandma around the block.... even kicking the dog creates some muscle and let's be honest, in today's world every little helps.
2) Secondly, and this is a big one, don't start cycling in a mountain range. I know it might sound all romanticized and what not but it isn't. It just sucks. It sucks a lot. But at least there is pretty scenery to enjoy while you throw up from overexertion.
3) Keep your luggage light. I know a 75 litre backpack might not look like much but it throws your bike a little off balance. I also strongly disadvise the carryance of oversized stuffed toys, while they may appear awesome (actually, let's be honest, they are awesome) when strapped to the back of your bike, they tend to get stuck in wheels and local kids try and grab them and well, they just aren't quite ideal.
4) Don't buy a chinese bicycle. The wheels tend to solidify with the body of the bicycle making riding a lot harder than it should be if not impossible. Also on occasion the handlebars tend to follow there own minds and duck and dive making breaking and gear changing a nightmare and this will conveniently happen while you're racing downhill.
5) Don't tell you cycle buddy that you'll meet up in the next town when they have all the local currency on them. It really does suck when you ride the last 30 km without a drop of water....
6) Don't cycle at night. Even if it is only 21 km in the dark... You tend to not realise that you have a flat tyre, especially when you don't have any lights (or you forgot to bring batteries) and then a sporadic thunderstorm breaks out and you get drenched and miserable and end up sleeping under a fruit stand on the side of the street where mosquitoes molestorise you and frogs don't shut up.
7) Take some tools with you. Even when you have a puncture repair kit and a spare tube and pump you might just realise at that inopportune moment, 14 km from the next town, that your wheels don't have quick release and have to push.
8) Buy gloves or grow a pair of man hands - blisters suck and you get to the point where you can't even change gears anymore. Gears are important. I know this now know.
9) Don't underestimate the ability to bruise... you won't know where they come from or why they hurt so much, but before you know it your whole body will be a bruise....
9) Don't start your journey without realising there is no physical way to cycle over 2000 km in 3 weeks... because at some point you're going to have to start hitch hiking to your friends wedding with your bike!!!