Hi,

They will use up essentially the same amount of calories.
Efficiency remains very near the same for calories burnt.

Note that to go 23mph needs ~ 50% more power output
than 20mph, (flat, no wind). So if you did a 10 mile time
trial at the two speeds, one would take 30 mins the other
26 mins, the latter using ~ 30% more calories in the process.

For the experienced and a fit newbie both doing a double
century you would both need less calories than the amount
the experienced rider would typically use on the ride.

The body maintains near the same efficiency for a wide
range of power output. An experienced rider can output
more power, more smoothly, under varied conditions
than a similarly fit cycling newbie - who generally
will vary output more in different conditions and
not be able to sustain as high an average output.

Very little energy is lost pumping extra blood to muscles
not used, as you need to do this anyway to sweat effectively.

Though as your legs develop inevitably more blood
relatively will flow through them more easily.

Runners would have an advantage over swimmers.

rgds, sreten.