talley if you like to follow "programs" there is a "couch to 5k" program. It is 9 weeks in length. I did the program for about 5 weeks and was able to do a 5K. Since you are already riding, this program maybe too easy, but it gives you a plan and other then some mild soreness, it protects you from yourself of wanting to do too much too soon.
Good luck with your first Tri. I have a long ways to go before I consider doing one.
chuckyd if you can do at your size, I guess I don't have any excuses.