I've seen a lot of information and discussion about what heart rate zones to train in for weight loss, but I am curious (or confused) as to what is best for improving my overall cardiovascular health, resting heart rate, etc. I'm 32 and not in the greatest shape (but not the worst either) and most of my riding time seems to be spent in zone 4 (90% +). For example, I just did a short 29min ride where my heart rate was in zone 4 for 73% of the time, zone 3 24% (however, my last ride before that was 18% z4 73% z3 over 39 min, and before that 86% z4 10% z3 over 59 min).
I don't feel overly out of breath or anything in zone 4 really, though sometimes I feel I'm becoming overly fatigued or fatigued too quickly (the Arizona summer doesn't help much, especially when my rides are usually mid day). I also realize the heart rate zone calculations used by bike computers (in my case, the Sigma ROX 10) can be subjective.
Should I try to ease up (something I find very hard to do) to see better improvements in my cardio health or is a lot of time spent in higher zones fine as long as I can handle it?