Be careful. I happened onto this thread, even though I gave up the idea of doing a tri. Besides the fact that swimming scares me and I'm a slow runner, I know I could probably complete a sprint tri now. The times wouldn't be pretty, but I would definitely finish. So why didn't I commit?
Well, I've involved in training, aerobics, cycling for about 25 years. I am one of those people who seems to have a much lower tolerance for overloading/stressing my body with huge amounts of training. I am getting over a very bad bout of bronchitis/asthma, etc that has had me down for almost 7 weeks. I'm pretty sure that this is a result of the winter training program I was doing. The cycling was at such a high intensity that it just breaks down my immune system. Then, when I started doing winter sports, that put me over the edge (just ask Deb W. how slow I was when we went skiing). I've had heart palpitations, sleepless nights, and a never ending series of allergic/bronchial things from over training. Then I get stressed from not exercising when I have to rest. It's a nasty cycle.
So, with work stress and the fact that I still have a couple of friends left who don't cycle that I like to see, sometimes I say "Today I am going to be a normal person," i.e. someone whose entire life isn't planned around how to get my ride/hike/yoga/weights in. You need a mental break sometime! Or, I go for a ride with someone who is slower than me.
Most people who don't train the way we do think it's an amazing feat. Keep it in perspective...

Robyn