Totally Awesome 30th

I've seen enough of "My Super Sweet 16" on MTV to know that my 30th Birthday will pale in comparison. However, I got to psych myself out. I want to do something out there. Something challenging. Something I got to work towards. So, right now, I have a couple ideas. My wife suggested going white water rafting. That sounds good, except it takes no preparation to do it. I was thinking... just thinking... maybe doing a 1/2 marathon or a full marathon (That's 13 miles or 27 miles). I am about 16 weeks away from my birthday. So if I start trianing now, maybe, just maybe I can pull this off. I went to one site for Novice's that has a 18 week training program. I may start fooling with it and hopefully know right off the bat if this is for me or not. Any suggestions? Any other ideas?