They say training on heart rate isn't the best idea but I don't have a power meter so I do exactly the same 😆
The standard setting for the top of zone 2 was 130 BPM but I've adjusted it to 135 that I just made based on how I feel and still be able to breath through my nose and talk in sentences (but not sing lol) that's the zone 2 check I learned from a Dutch podcast.

For my zone 2 rides I've set up my Garmin so that it doesn't display my speed on the main screen, I had the same problem you're having but now I can't see how fast I'm going it's a lot better and usually I can keep around 132 without looking on my screen, when I forget Garmin notifies me at 136BPM

A 4 hour training ride is serious shit, even in zone 2. Good luck with your plans, 100 miles... OMG!

I want to break 100 kilometers somewhere this year on my MTB since I don't have a road bike (yet 😁)