As much as I love training (well, usually), I don't think I could stay excited about it if I wasn't building up for something. I love having a measured, timed course where I can run my hardest, try to beat previous times, and be part of a larger community (since usually I run on my own.) Also, in the case of the marathon, I don't think any other running accomplishment can compare to the feel of finishing that sucker. (1 so far, working on #2.)
I've done 5k, 8k, 10k, and a marathon--strangely, never a half. Like I said, the thrill of the marathon was incomparable. Other than that, the 10k is fun--not so long that you really have to suffer, but you've certainly had a good run at the end of it. I do like 5s and 8s too, but I'm maybe not quite as crazy about the 5k because since I'm now able to do it pretty fast, I feel that I should practically sprint the course. I was very pleased with the result of my last 5k (21:04 chip time), but boy were my legs burning during it!
In high school I did at least eight races a year, did just one a couple years later, then last year I was marathon training and did a 5k, 8k, and 10k in preparation (and as interim goals, to keep me motivated over a long season.) I've only travelled out of town for the marathon, and for one or two races in high school.