I ran a 5k last month and it turned out to be one of the highlights of my year! I ran/walked 3 days a week for about 2 months...then I stopped working out for 2 months...then I thought "I better start taking this seriously", so I took a jog the day before the marathon. I don't suggest my training regime.
All that said, I surprised myself by jogging the majority of the race. The whole experience was awesome, but I really regretted not training for it--I KNOW I could do so much better.
My goal for 2012 will either be to run a 27-minute 5K marathon OR to do a half marathon (no time limits on that one--just surviving will suffice). I will probably settle for the lesser of the two evils, but I haven't figured out which option that would be