I am not a professional painter, but I've done both interior and exterior painting on the houses I've lived in before. Also, I realize the original post is kind of old.
To reiterate something many other people have said: when you think about painting a room, you see yourself standing there with a brush or roller, putting the new paint on. That step is maybe 25% or less of the work, and comes fairly close to the end of the whole process. You will spend a lot of time before that cleaning, fixing cracks, and other prep work. You'll wonder why you're doing all of that work, but it helps you to spend less time actually painting, and get a better result.
Another kind of psychological thing is that it will look worse before it looks better. Right now, that interior wall is an out-of-fashion color, and maybe it's a little dusty or cracked in a few places, but it's more or less all one color - you could picture somebody living in that room with the wall the way it is right now. But once you dust it, fix the cracks, maybe scrape a peeling spot - it will look worse than it does now, with spackle over the cracks, and "bare spots" where you scraped it. It's easy to feel like you're going backwards. Once you get the first coat of new paint on it, this feeling will mostly go away.
For the interior, if you have anything else to do to the walls you are going to paint - like hanging up a curtain rod, or replacing an electrical outlet, or anything like that - do that before you paint. 9 times out of 10 this doesn't really matter. The other 1 time, when you drill holes for a curtain rod, you find an old nail or drywall anchor you didn't know was there, and either make a big hole, or have to move the thing over 1/2" and redrill. Or, when you tighten the screws down that last little bit on the new outlet, the drywall cracks under the "ear" on the outlet. Then you get mad because you have to fix the hole or crack, and get the paint stuff out again and fix the new paint. If you do all this stuff before you paint, the new crack or hole is easier to fix when you're patching the rest of the wall too.
For the exterior, if it needs an extension ladder, and you haven't used one before, look up some "how to ladder" videos online. Also, try to use the ladder when there is someone else around. You don't have to have someone else there at the house you're working on - it can be a nearby neighbor that is home, or the city street crew that is working down the block, or even the (older) neighborhood kids that are riding their bikes around. Their job is to call for help if you fall off the ladder.