It sounds like you’re right on the money with your current volume and intensity.
Here are a few thoughts:
* Don’t worry about under-doing it. It’s far less costly than over-doing it.
* It’s normal for HR to be a lot lower on descents. Don’t worry about trying to get the same heart rate as ascents. To do so would likely put too much muscular strain on your quads, which can take a long time to recover from because of the different type of loading (eccentric versus concentric).
* If you’ve had a metabolic test done, you can ignore the zone prescriptions in the book. Use the zones from your test.
* If the top of your Zone 2 is less than 90% of the bottom of Zone 4, then you can do most of your training in Zone 2. If you’re at 90% or higher, then you’ll want to mix in a lot of Zone 1. (To calculate the percentage, divide the high-limit bpm of Zone 2 by the low-limit bpm of Zone 4 and multiply by 100.)
* Because you said you “feel great after each hike”, I think that you can probably increase the duration or frequency of these sessions. Start with 5-10% per week for a couple weeks and see how it feels. Try and keep that “feel great after” feeling.
I hope that helps. Let me know if you have any other questions.