Since this gets asked often enough, I decided to post this quick list:
Pace
Slow down, if you can't speak in short sentences you are going too fast. If you are getting side stitches, you are definitely going too fast. As you rack up the slow miles, you'll notice that your "slow" pace is going to increase by itself. I have surprised myself many times when my Garmin gives me a lap reading a minute faster than what I was feeling, and even in hilly terrain.
Distance
Avoid increasing your weekly mileage by more than 10% per week until you understand how your body handles the added abuse. I used this rule to go from 5 miles/run to my first half-marathon in about 4 months. And I took me less than a year to go past the 20-mile mark. At 21.5 miles I was fine, energy was fine, but my feet hurt too much even with maximalist shoes (this is on a fresh pair of shoes, I rotate between 3-4 pairs). I can run 70 miles per week (I have experimented up to 90/week) without hurting myself, that's 6 running days, one rest day. If I keep that schedule I can run hit half-marathon distances about once per week without much drama. Going past 14 miles in one run is just painful.
Fuel
You'll eventually hit glycogen depletion, everyone is different. Your lungs will be fine, you won't feel any pain, but you will feel like a dry sponge, like you have zero energy left to move. This happened to me somewhere between miles 10 and 13, I started experimenting with energy gels (100-120 calories per portion) and that got me to 21.5 miles. I currently take an energy gel every 45 minutes whenever the run is longer than 10 miles.
Mental game
It's great if you like music but having to maintain situational awareness means not being able to listen to it as loud as I'd want. And an annoying side effect of running long is that Bluetooth headphone battery life varies wildly with temperature conditions, so you end up carrying a second pair just in case. I also use my runs to think about work problems (I'm a software developer) and life problems in general. Another thing that helps is that I avoid straight line routes like the plague, if you look at my routes you'll notice that apart from the Washington & Old Dominion park trail, I almost never hit true straight routes. I also try to add randomness to the routes just to confuse my muscle memory.
The 10-minute rule
If I am not sick, and I am not hurt, and don't want to go on a run, I still go out and commit myself to a 10-minute run. The worst case scenario here is that I'll quit after 10 minutes, which means I have to turn back and end up having a 20-minute run on a day I was going to skip. If after 10 minutes I am not sure, I run another 10 and reassess. Many times I have been able to salvage the run just by taking it 10 minutes at a time.
