Beth has encouraged me to share my triathlon experience too, so briefly here it is.
My sister-in-law, Jill, and I finished the Little Miami Triathlon yesterday. My time was 3:48. Definitely not fast (the winners are around 2:00), but a 29 minute improvement over my time last year so I was happy. Of course, I'd still like to do better! :)
Here's a good online source for training programs: Tri Newbies. As a stay-at-home mom, it is definitely hard for me to find time to train for events or exercise period. I'm often on the treadmill at 9 or 10 pm. Moms tend to put their health last, which is a mistake.
Don't ever tell yourself "I could never do that" about anything. Find something to challenge yourself, whether it is an exercise class, a 5K, etc. Start small and keep pushing. There will always be someone faster, stronger, better, but don't ever let that stop you from doing all YOU can do. Unless you push yourself, you'll never know what you can do.
I never thought I'd be a runner, but I'm getting there. When I started training last year, I could run about a mile. This year (after pretty much taking the winter off), I'm up to 7 miles. I'm hoping to make it to 13.