Time Trial Strategy

Help!
I have the Nationals Time Trial Race to do tomorrow which was supposed to be a 40K flat race. It's still mostly flat with some slight uphills, a tailwind in the beginning and a headwind on the way back. The race is run over a 22.5 km loop done twice which increases the distance by 5 kilometers more than I expected.
Given that I only train 3 times a week and lack the endurance to maintain a strong effort for that long, does anyone have a good idea of what kind of strategy is best to use with HR monitoring?
For training I usually do an uphill time trial which I start at 170-172 bpm for the first 25 minutes, and then (it gets much steeper) I raise the HR to 175-180 which I maintain till the end. The whole thing lasts about an hour with my best time so far this year being 51:30.
My best time for a 30Km Time Trial is 45 minutes (a 40 km/hr average at an average HR of 177). My Max HR is 194bpm.
Is it better to start the first loop at 173-175 bpm and then raise to 177-179 or will it be too slow? Or is it better to hit a given HR (say 175-177) and attempt to maintain it throughout?
Any comments from people's experiences will be greatly appreciated.
Thanks
George K.