My own uni

Average speed based on wheel size over longer distance will vary a lot based on skill, crank length and road conditions. A 26" at 10 mph would be cadence of 129 rpm and that is quite fast.

From a past thread one poster wrote:
Estimated/Actual Averages over long distance (miles per hour):
Coker (35er) …12-14 mph (from threads on this forum)
29er…8 mph (me, out for a nice ride, 125mm cranks)
29er…6.5 mph (me, racing crosscountry/MUni, 150mm cranks)
26…7 mph (est)
24 MUni…5 mph (me, racing crosscountry/MUni with 170mm cranks)
24 MUni…3 mph (me, casual)
20… this is too small for long distance. (IMHO)

And another posted:
A Coker does 1mph for every 9.34 rpm
(93 rpm = 10 mph)

A 29 does 1mph for every 11.6 rpm
(116 rpm = 10 mph)

A 28 does 1 mph for every 12 rpm
(120 rpm = 10 mph)

A 26 does 1 mph for every 12.9 rpm
(129 rpm = 10 mph)

A true 24 does 1 mph for every 14 rpm
(140 rpm = 10 mph)

A 20 does 1 mph for every 16.8 rpm)
(168 rpm = 10 mph)

Figures are approximate as most wheel sizes are only nominal.

So, what is your sustainable cadence? 60 rpm = 2 pedal strokes per second. 90 rpm = 3, pedal strokes per second. 120 rpm = 4 pedal strokes (two complete circles) per second.

For me as relatively new 36" rider over 20 miles I should be able to average 8 to 10 mph with my 127mm cranks on a relatively level smooth route. Everyones mileage will vary.