How did you determine your heart rate zones for the run?

Go do a HRmax test and redo them.

It may be safer to do a 30 minute LT test. I usually do it on the treadmill and here is how it goes:

1. Start with a good 15-20 minutes warm-up take it easy and slowly pick-up the pace
2. Start the test by running 10 minutes at pace that you can sustain for 30 minutes. After 10 minutes segment press lap button on your HRM.
3. Run another 20 minutes at the same or slightly higher pace, but be sure you do not slow down. Pace yourself so you can finish the 20 minutes without any troubles, but you should be reasonably exhausted at the end so you are not ready to run much further. At the end of the 20 minutes press lap button on your HRM.
4. Cool down for 10 - 15 minutes and stretch

After the test your run LT should be the HR average of the 20 minutes segment. Then go to the book store and look up Total Heart Rate Training book from Joe Friel and look-up your HR zones from there. Joe has split zone 5 into 5a, 5b and 5c which may be useful if you do a lot of speed work. I do follow the pacing recommendations quite rigorously in my training and it seems to work quite well. Knock on wood I did not have any injury since I started this approach in February and I was sick only once (quite an achievement with 2 years old son in day care). I found that the 220 - x formula did not work for me. The LT approach works much better and now after doing this training for a while I can easily determine my pace and corresponding HR on the spot.

That's my 2c ;-).

Jan