Recently I have been averaging 17 to 18 mph for up to 54 miles (~3 hours).  Below is a  record of recent sessions:
 
Date: Day: Trail or Curcuit: Distance: Weekly Cummulative:
May 29 Sat Red Cedar* 30.0 30.0 Miles
May 30 Sun Elroy-Sparta 60.0 90.0
May 31 Mon 
(Memorial Day)
Cannon Falls 40.0 130.0
June 2 Wed Gateway, MN 44.0  176.0
June 4 Fri Glacier, MN 26.0 200.0 miles
June 7 Mon Red Cedar / Chippewa* 52.0 52.0
June 9 Wed (humid!) Red Cedar / Chippewa* 54.0 106.0
June 12 Saturday AM Gateway, MN  44.0 150.0 miles
June 13 Sunday Baldwin-R.F. 51.0 51.0
June 14 Monday Red Cedar/ 
Chippewa*
52.0 103.0
June 16 Wednesday Hudson Group 28.0 131.0
June 17 Thursday Red Cedar* 34.0 
(17.1 mph ave.)
165.0
June 18 Friday Red Cedar* 35.0 
(17.4 mph ave.)
200.0 miles
June 20 Sunday Cannon Falls 50.0  50.0 
June 21 Monday New Richmond 20.0 70.0
June 23 Wednesday Hudson Group 30.0 100.0
June 24 Thursday Plank Road, IL 20.0 120.0
June 25  Friday  Plank Road, IL 46.0 166.0
June 26 Saturday Plank Road, IL 34.0 Hot! 200.0 miles
June 27 Sunday Red Cedar* 35.0 35.0
June 29 Tuesday Local Circuit 30.0 60.0
July 3 Saturday RF/Diamond Bluff Circuit 50.0 Hot! 110.0 miles
July 4 Sunday Red Cedar/ 
Chippewa*
72.0  Hot! 
(14.1 mph ave.)
72.0
July 5 Monday Gateway 28.0 100.0
 
July 7 Wednesday Hudson Group 30.0 130.0
July 10 Saturday Red Cedar/
Chippewa
42.0 172.0 miles
July 12 Monday Red Cedar/
Chippewa
 
 

  *Red Cedar is a crushed limestone trail.