I have only just found this thread and have read all the messages. I competed in my first Tri in Derby in september. Had a great day, completely knackered at the end of it but had fun. People of all abilities were there, and yes all types of bikes were used. I passed one woman on her bike riding along like miss daisey. Some Tri's are held in swimming pools so no veils desease to be caught. Lots of tri's allow team entry's and two good web sites to look at are:
www.britishtriathlon.org
www.220magazine.com
both will give you listings of up and coming events, icluding the 'rowathons' you have been talking about.
give one a try. you will be hooked. promise.