If it hasn't been made clear from the other posts, your maximum heart rate is WAY higher than the default values the Garmin calculated for you. Mine is as well, although not as extreme as yours. So the computer is saying you are in Zone 5, when in reality you are likely in Zone 4. Based on what you've provided, your max is at least 205, maybe a bit higher. So your 181 average for 2 hours is near the top of Zone 4. Holding a high Zone 4 for that long is a hard workout.
If you haven't already, create an account at garminconnect.com. There is a zone calculator you can use, or you can enter your own custom zones (like from Friel's book). Either way, you then download them to your computer, and from then on, your zone data will be more accurate.
I've done the self testing, but am still not sure I have my zones nailed down, so I'm having a metabolic test done to be sure I have it right.
Edit: I just noted your age, so your max isn't that far off the formula calc - but you still need to get accurate zones into your computer.
Last edited by AzTallRider; 10-28-10 at 05:47 PM.
Reason: I had missed something