Efficiency for "more speed" vs "longer distance"
More speed, for shorter elapsed time
--Aero on the flats
The biggest free speed increase is getting your bike is fitted so you can comfortably use the drops. (Well, getting much stronger would work, too.)
--Harder efforts on the climbs.
Riders can't make up time by going faster on the downhills. The time saved with hard efforts on climbs is significant.
For example: a hill climb and the same descent: A rider climbs at 6 mph for a mile, then descends the mile at a prudent 30 mph. 10 minutes for the climb, 2 minutes to descend, 12 minutes total. Even at an impossible 60 mph downhill, that's still just a 1 minute savings. Doing 8 mph on the climb instead of 6 saves 2.5 minutes -- that's a significantly harder climbing speed, though, around 20% more power.
--A once a week interval training ride.
Even an informal, shorter ride with very hard intervals and recovery in between. It probably doesn't matter how long the intervals are, either. This will boost your maximum power for the climbs, and help overall.
Longer distances
Like other posts said, it's more about steady efforts, avoiding "burning matches" and knowing your all-day perceived effort / heart rate / wattage.
Aero helps here too!
Training with longer rides to get used to the long saddle times.
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Aero
The required power goes up by speed cubed. See
this old thread for the physics.
So, going from 17 mph to 20 mph on the flats,
this calculator, taking the defaults with hands on the tops, says the power is 153 watts at 17 mph, 239 at 20 mph, 50% more power! Even a half mile per hour increase is significant, and takes noticeably more power.
From the same calculator:
Hands on the tops at 17 mph: 153 watts
Hands in the drops at 19 mph: 153 watts. Aero really helps.