I did my first tri (Sprint of 1/4 mi swim, 30k ride, 5k run) this summer on short notice with maybe eight weeks of training time. I signed up because it was more of a "I bet you won't..." sort of thing with my wife (who needed to be proven wrong) than something I had really anticipated doing. It was the most fun I've ever had in my life!
I'm not built like an endurance athlete (5'9, 185) and I didn't train as well as I should have, so I had an uphill battle. But I set a goal of a sub- two hr time (I picked two hours because I knew a few people who had done sprints in approximately that time and figured I could hang with or beat them) and ended up crossing the finish at 1:57:41.
The swim was without a doubt the most difficult part. A quarter mile doesn't look that far, but when you're an inexperienced swimmer and you're swimming in traffic, in a lake, for the first time, it SUCKS. I thought I was going to die, and that thinking messed with my T1 time. Experience really is the best teacher, because the next tri I do will have about two mins cut out of my first transition.
There are a lot of different options for first time triathletes or for those interested in expanding into endurance competitions (shorter distances, duathlons, etc), so if you think it's something you want to do, SIGN UP AND DO IT. Life is too short to be left wondering if you could or not. I'm hooked, and can't wait for next Spring/Summer.
First post... woo hoo.