Looking back at my records . . .
IME 5600' 200ks are completely different animals. They're easy enough that I could ride them more like a century:
Z1 : 3%
Z2: 20%
Z3: 46%
Z4: 27%
Z5: 3%
= 7:21 saddle time
300's with 10,000' are much more demanding of endurance:
< Z1: 14%
Z1: 45%
Z2: 37%
Z3: 4%
= 12:02 saddle time (195 miles)
A flattish 400 (5000') is about the same:
<Z1: 7%
Z1: 46%
Z2: 32%
Z3: 10%
Z4: 5%
= 13:17 saddle time
A 400 with 12000' is harder:
>Z1: 21%
Z1: 43%
Z2: 30%
Z3: 6%
= 15:37 saddle time
I was over 60 when I did these, so younger folks should be able to run a bit higher. I never was a fast rider, just well trained and steady. The fast boys were a couple hours or so under these times.
For long endurance rides, just gear down and keep your butt in the saddle. No popping over the little hills. Keep the effort down unless you have to because it's really steep. The fast boys do the same thing, they just use bigger gears.
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Results matter