Using a Road Bike for an Ironman 70.3

Road bikes are fine for any distance triathlon.  You will see quite a few of them out there on the course and there is no physical or mechanical reason why you can't use one.  A tri bike will help put you (the rider) in a more aerodynamic position which can save you a good chunk of time, but it is by no means a requirement.  You still need to pedal the bike.

One option you could do is get a set of short "jammer" or "shorty" style aero bars that were meant for road bikes.  This will at least give you the option to ride in a slightly more aerodynamic position for periods of time without compromising your traditional road bike position.

Edit: Just saw you already have aerobars on your road bike.  Most should be fine, but if I would choose the shorter versions if I had not done so already.

^^^ this ^^^

Before the tri bike arms race hit full throttle (about 2010), "most" people did half and full IMs on road bikes.  I did my first IM back in 2006 on a Trek 5200 road bike with jammer style aero bars. Probably 2/3rds of the field were riding road bikes. 

I did get myself a Cervelo P3C as a reward after finishing that first IM, and while I train and race on my tri bike I of course still have the same road bike and ride it all the time.

Honestly a large portion of the field would be better off on road bikes than their near upright fit with aerobars.

Shorty bars are designed for road bikes, and with road bike geometry and fit in mind.  Basically, it's just a place to rest your forearms and hands so that your arms are not as far apart like they would be while you're riding in the drops.  

The reason I recommend this is because it allows you to get slightly more aerodynamic without compromising your road bike fit.  Once you start using fast forward seat posts and more traditional aerobars...well...then your road bike is no longer a road bike as you have altered the way you are fit when riding on the hoods and in the drops.  Basically...you traded two good road bike positions for one semi good aero position.  If all you're using your road bike is for triathlons...then that's fine...but you may also want to consider selling the road bike and just getting a tri bike.  But if you do plan to ride your road bike like a road bike...then I would advise keeping your road bike position intact...and just adding shorter aero bars that can be used comfortably without adjusting your road bike position.

That part is pretty key for people that are doing a time trial and crit/road race on the same day.  Further you can get into a pretty solid position with the shorty bars:

For me I just put the shorty bars on, and I'm good to go!

~jhaak

If you look at msteiner's photo, you notice that his hands are not that much more forward from where his hoods are.  IOW, it's really not that much different than if he had his hands on his hoods, but kept his forearms straight (or if he was in the drops) as far as how being on those bars impacts his fit.  He's more narrow on those bars, so that improves his aerodynamics while keeping the same road bike fit.  So like he said, you basically just slap them on and you're good to go.

The difference with more traditional clip on aero bars is that they are designed with longer extensions, and encourage you to have your elbows further up as well as your hands.  This is what stretches you out, and thus you feel like you need a forward facing seat post...which then in turn compromises your fit while riding the hoods and drops.  

Of course you could always use traditional (longer) aerobars but just stay back on them...but why have all that extra unused extensions if you don't need them.

When I first started out on a road bike, I added Profile Design T2+ clip ons.  They are more traditional aerobars, but also highly adjustable.  I told the guy fitting me that I didn't want to adjust my road bike position, just make adjustments to the aerobars the best you can.  He slid the extensions back, as well as the armpads...as much as he could without it hitting my knees.  I suppose he could have trimmed them to make it an even better fit, but I knew it was just a temporary thing.  Shorty's would have been a lot simpler.  

For added reference, here is what I use:

http://www.amazon.com/Vision-Team-Mini-Clip-Aero/dp/B005VZUX6K

That's just a ridiculous argument.  How many pros do you see riding road bikes at Kona nowadays (or in the past 15 years or so)?  That answer would be zero.

And they didn't use tri bikes back when Scott was winning Kona because Dan Empfield had not yet designed the first tri bike.

Very true.  I see people buying $5000-$10,000 tri bikes with godawful, upright positions all the time.  They would be better off on a road bike,