LTHR Bike Test

Did my LTHR test this morning, this would be the second test in the last 3 months. Set it up as almost identical to the last test, the main difference was I was on my tri bike. I did not go aero at all, so basically the same as my roadie. 15 minute solid w/u with some spins to get the HR up. 30 minutes on, as hard as I could, took the average of the last 18:39, I did not make the entire 20:00, was done, gone, just bout puked. Was in the biggest gear I could handle keeping a minimum of 90 RPM. I was on my CycleOps fluid 2, just like last time. And, I came in at the exact same 159 BPM avg. My question is: I know the test was done as well as I could, rode as hard as I could, and would have figured that I should have raised my HR a bit since last test. I do a fair amount of LT training. And at this HR, I was just worked. I have ridden the flats and some slight downhills at 26-28 MPH, HR at 150 and barely breathing. At 159 during the test, I was gasping for air, still working, but extremely oxygen deficient. I figured the trainer was thee best way to duplicate the test, but am wondering how accurate it is. Comments, suggestions please. I am consistant and persistant in my training, my next A race will be my first HIM in September and the first half of the bike is basically all climb. I will finish, God willing, but am wondering how competitive I will be.