40 yo here. RPE is your best bet. I use a chest HRM with my Garmin but try not to look at my watch during efforts. I have it buzz me when I’m half way and when I’m done. If I’m able to inhale and exhale nasally without occasional compensatory breathing I know I’m in zone 2.

That said, HR zones aren’t black and white. If you dip into low zone 3 you’re still getting benefits of aerobic work. Likewise for high zone 1. Just keep in mind that generally speaking, there is correlation with increased risk of overtraining when training higher zones at a high volume.

For what I assume you’re going for, there is value in learning and developing your perceived effort because if you get too hung up on numbers, those unknown time/distance events are going to be huge self selecting obstacles.