I'm doing my first duathlon at the end of June. 2.5/10/2.5 Hunstville, AL. I have alot of running experience but not so much biking. I've pulled out my 22 year old road bike that I got when I was 16 (1989)!!! It's a Giant Perigee. Dropped it off at my local bike shop for them to give it the basic tune up. I have cages on the pedals so plan on running in my running shoes and biking in them, obviously. The tech at the shop started giving me this speech about upgrades that I needed, particularly something about a different crankset, blah blah. Something about better leg motion. Really? I just want to survive this race and see if I even like doing a multisport event. Can I get by with the bike I have. How ridiculous am I going to look? Do beginners use bikes this old or am I way out of the league on this?
I'm almost certain that bike would do just fine. Best way to tell? Go out and ride it for 10 or 20 miles.
I'm a strong proponent of doing exactly what you're suggesting, trying a few races as cheaply as possible. Lots of people at the race will likely have older road bikes or hybrids or mountain bikes. Of course, lots of people will also have bikes that are ridiculous but remember everyone starts somewhere and your equipment doesn't decide who you are as an athlete. If you like multi sports, then decide if its wotrth money to put in to the sport.
I have a feeling what he is getting at, is it likely has "biopace" chainrings, which are ovalized in what is now considered the wrong manner(it's easier to push down, and a little harder to "pull" over the top based on the ovalization). But honestly, lots and lots of people rode lots and lots of miles on biopace chainrings in the early 90s. Eventually they fell out of vogue. However, if they shift fine, then I'd leave them.
My wife did her first duathlon on a 1989/1990 Raleigh Tri Comp with biopace chainrings and she did just fine.