If you can average 147/150's bpm for 1 hour 6 minutes riding fast you should be able to average higher than that for an all out 20 min time trial test effort unless you are on some medication that puts a governor on your heart rate like beta blockers. Your zone are set too low based on a threshold heart rate of 147 bpm.
Here is a Training intensity guidelines table detailing what each intensity should feel like.
I recommend getting your HR zones figured out before you use HR as a pacing tool in a race.
Here are my guidelines how to race a PR 50 miler.
Lynda