Road cycling with a heart rate monitor
I'm trying to get educated on how to use the heart rate monitor that I am planning to buy soon for road cycling. I am 63 and ride about 45 miles about three times a week with two friends. Based on a treadmill stress test, I think a very conservative guess at my maximum heart rate is about 150.
If zone 2 (aerobic endurance) is about 65-74% of max HR; zone 3 (aerobic threshold) is 75-84% of max HR; and zone 4 (lactate threshold) is 85-91% of max HR, in general terms, how much time should I be spending in each of these zones over the course of 45 miles? I'll start getting specifics soon, and I could be way off, but I'm guessing that I currently spend at least 20% of the ride in Zone 2 (easy talking), no more than 10% of the ride in Zone 4 (hills), and about 70% of the ride in Zone 3 (comfortably hard). So I'm guessing my heart rate is probably averaging 120ish (150 x 0.8) in Zone 3. If that guess is accurate, is that too much time (~70%) in Zone 3? I really enjoy pushing myself but don't want to overdo it without understanding what I'm doing. E.g., I have heard a lot about the downside to strenuous exercise for those in my age group. Thank you for any help you can offer.