I am 31, and have a very similar "profile" to yours, including the herniated L5/S1. I just decided that I'm going to train for Ironman 2010... maybe 2009.
I, too, am just interested in finishing, and feel like running is my weakness, although I have completed one marathon and several half marathons. I say go for it! You won't know til you try, right? And the worst thing that can happen is that you get hurt... but you could get hurt every day, doing what you normally do. Listen to your body, train smart, and have fun!
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