More Time Trial questions

[Jiffer]

For those of you who do time trials, how do your tempo ride times compare to your actual TT times? I am working toward doing my first TT. I do 20 minute tempo intervals on a big loop near my house with 155 feet of elevation gain. The farthest I have gone is about 7 miles in 20 minutes (20.7 avg pace) and the first TT I do will be 12.5, so close to twice as far. And it actually might be exactly twice the elevation, depending on which TT I do first. Assuming I did that one, a 12.5 mile TT with 300 feet of climbing, is it reasonable to assume I'd do it about the same average pace as my tempo ride? Or might it be faster because of race day adrenaline ... or slower because it's a longer distance?

I know wind and other things play a big part, but generally speaking how would you say your tempo rides compare to your TTs? I don't have a power meter and still working on getting my stupid HR monitor to work ... so I average pace is really my only method of analyzing how I'm doing.

Also, would you say it's important to do a full 12.5 mile mock TT before my first? Kind of makes sense, but I also know you don't have to ride 100 miles to train for a century or run 26 miles to train for a marathon.

Thanks!