You got some great input from TxDoc, but I'll add a few comments too.

Quote Originally Posted by TxDoc View Post
Short answer, they don't.
Long answer, see below...
Agreed. First, tempo rides are, well, ridden at tempo. Typically in a TT, my efforts are high LT and into anaerobic. For example, my max HR is about 182. For a TT effort, my average HR hovers around 177, with a max typically around 180. On a tempo ride, my average HR is in the 150-160 range - much lower than at TT effort.

Quote Originally Posted by TxDoc View Post
In time trial you expect to go as hard as you can. This means, as hard as you can go over that course, using all you have, with a reasonable steady pace. It is a hard effort. You will probably want to fall off the bike at 20 minutes, curse the sport and the race at 30, and be about to throw up at 40 minutes. If you are happily maintaining a fast pace and feeling good, chances are you are not going as hard as you can go. Old school crono specialists used to say that if it does not hurt it is not time trial.
Absolutely! No doubt, a tempo ride can feel like a hard effort, but a TT will feel worse. It hurts, and this is where the mental part of it comes into play. You need to be able to mentally push yourself harder than you think you can. When I cross the finish line, I can barely stand up, can't talk, and often, feel like throwing up (yes - it IS fun ).

Quote Originally Posted by TxDoc View Post
One small recommendation first - start thinking about a power meter. You do not need one to race, but if you want to specialize in time trial, it becomes avery useful piece of equipment. That said, yes, there are a few things you can estimate with speed and/or HR - as long as you keep always in mind the variability of speed and HR and can take the fallacy of your measurement system into account.
Here's an alternate opinion - I don't train with power. Never have, and am not about to start. I do know what my wattage zones are, but none of my wheels measure power. While not as accurate for MANY reasons, I train by HR and have been doing that for over 5 years. BUT, I do agree with TxDoc that you need to test to see where your zones are - if you are simply going by RPE, that is ok, but know that your efforts are likely not quite as hard as you may perceive them to be.

Quote Originally Posted by TxDoc View Post
So, back to your TT - there are a few things specific to time trial which is why the time trial specialists train time trial and often try to simulate race conditions to predict how things will go during the race. My recommendation would be:
1-yes, do a 12.5 miles TT, and do everything like you would on race day (same nutrition, setup, bike and wheels of course, same warmup, etc).
2-if possible do a recon on the race course and try a TT there (will help determine entry/exit lines around dificult corners or turnarounds, areas to watch for, any problems with the road, etc).
There's a couple of 'first TT' threads on the 33 on bike forums, good advice so go take a look.
Good luck!!!
Personally, I think you are spending too much time stressing over this before you have even tried it! I showed up at my first TT with a too-big-for-me road bike with MTB pedals, and a kit that was baggy. I had NO idea what I was getting myself into - had never raced on the road, and CERTAINLY had not been training. I had pre-ridden the course the previous day, but that was the entire extent of my preparation. I finished 2nd in my age group that day, and have been hooked on the TT ever since.

You are going to be FAR more prepared than many of the people who show up. Relax, and have some fun.

SheFly