OP, to know what your endurance pace is, you must first establish the baseline: your lactate threshold. Get a HRM, and do a simple LTHR test. Ride for a little while to get warmed up, then go as hard as you're able for 20-25 minutes. I'll tell you right now-- it won't be fun.
After the ride, look at the average HR during that 20-minute period, that's your threshold heart rate. Set your zones from there using any of the numerous zone calculators.
Most rides will be spent predominantly in Zone 2, or even a split of Zone 1 and Zone 2. Efficiency comes from conditioning, IMO. I do rides with average speeds 2-3mph faster than when I started cycling, but instead of an average HR of say 145, it's now 120.
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