1) Bring tools to fix the most likely mechanical problems. For really oddball mishaps, you can hitchhike to civilization or find a road angel to give you a lift. I've never had a tire fail in 40 years of cycling. I start a tour on good tires and don't worry. If they're looking old and cracked, I replace them before the tour starts (but I don't usually let my tires get like that.) However, I've had plenty of flats, so I bring a spare tube, a patch kit, and tire irons. Learn how to fix a flat reliably (if you don't already know), especially how to reliably find the thing that caused the flat in the first place. I've had tubes that were unpatchable - usually because there's a tear around the valve stem - but it's unusual. If it happens once, I've got the spare tube. If I get another flat before replacing the tube I can patch it.

I've had spokes break, and it's much more likely to happen on tour when you're carrying a big load. For that reason I carry Stein Hypercracker tool and a couple of spare drive-side spokes. I also carry the FiberFix spokes as a backup, since they're so light. Of course, you need a spoke wrench too. Since I started carrying this stuff I've never broken a spoke, but I feel much more secure knowing I can deal with one if it happens.

I carry a multi-tool. Sometimes I have to tighten bolts that have loosened. Sometimes I just tweak adjustments on things - saddle height, handlebar rotation, brake lever position, etc. Sometimes I have to adjust the limiting screws on the derailleurs. My multi-tool has a chain tool. I've never had a chain break, but I've seen it happen. It wasn't hard to use the chain tool to get the bike rideable, albeit with fewer gears available.

I've had bolts vibrate loose (almost always on racks) so I use Loctite when I install them, and I carry a couple of spares - cheap insurance.

I also carry a few zipties and spare brake and derailleur cables. Zipties have come in handy dozens of times for all sorts of obscure uses. I bring a bunch. I've never needed the cables. If they weighed much I probably wouldn't bother, but they're light so what the heck?

2) Be prepared for a huge increase in your appetite. I probably eat close to twice as much on tour as at home - two breakfasts, big snacks throughout the day in addition to a big lunch, and a big dinner. I know there are people who aren't affected this way, but I've also talked to lots of first-time tourers who were amazed at how much they were eating. I spent a lot more money on my first big tour than I had expected, and the biggest reason was the amount of food I was eating.

3) You'll probably bring stuff that it turns out you don't need. You can send it home. I've done it several times - even after years of experience. Go as light as possible - ounces add up, and extra weight makes hills seem steeper and can contribute to breaking spokes. That said, don't go so Spartan that you're uncomfortable and miserable. Comfort items seem even more valuable on tour when you're doing without so many conveniences.

Whatever choices you make, don't lose sight of the fact that you're doing it for fun. If you're starting to suffer, why not take a day off? If you find a beautiful campsite that's 20 miles short of your hoped-for daily average, why not stop? Enjoy! If you are really into your book and don't want to stop reading, why stop? So you get a late start, so what? It's your trip, not someone else's. (Of course, if you're travelling with someone, all of those points are up for negotiation.)

Have fun!