A 3-4 hour ride is usually zone 3 and a bit into zone 4/5 depending on terrain for me. If I'm in zone 2, it's normally a 100+ mile ride for me (and even on this weekend's 109 mile ride, 25% was zone 3, but it was hilly and there was a bit of headwind). The double century I did last summer was nearly all zone 2 as it was something I needed to keep up all day (just shy of 14 hours of riding in the end) if I was going to finish and finish before my goal of sunset. I couldn't risk blowing up too early. Socialble rides with friends helps with pacing.