So I'm quite new to pacing and HRM in general but I've read a lot to start out and gained some experience for the past two weeks. I measured my resting, max and functional threshold HR and set up my zones based on Joe Friel's book. (So Zone 5's lower end is my FTHR).

What is the right HR zone for a very long, very challenging endurance ride with a lot of climbing? In theory I know that I should pace myself to stay in Zone 2 and only let myself up to Zone 3. But during steep climbs this seems to be impossible. If I climb very slowly, I'm still at the lower end of Zone 4 and it feels very-very slow (and it's super hard to lower my HR to zone 3). Is it risky to stay at the border of Zone 3-4 during very long climbs (45-70 min)? Or stay entirely at the bottom of Zone 4? Would you start out slower or faster during the ride?

When I'm on a long climb (~1 HR) full speed, I have a heart rate floating around my FTHR (95%-105%). When I'm on shorter climbs I like to push myself and go beyond, sometimes hit BPMs closer to my max and let myself recover to the FTHR area. Probably this is why it feels so slow and weird to climb in lower zones..