Interesting. On my errand rides my iPhone Cyclemeter app usually records ~12 mph clock running over largely residential streets, but there are so many variables that "average" is not a useful tool. Eg, yesterday's 12 mile return leg recorded 11.9 but that included a 5-to-10 minute stop to adjust a cleat on the first-time-this-year-broken-out Lake winter boots, as well as 3 1/2 miles of climbing against a stiff N wind in a 75" gear on the gofast (didn't walk on the very steep 4/10 mile hill up the Rio Rancho mesa! Light wheels **climb*! Whoo hoo!). Outbound, with the wind behind me and 500 feet of elevation drop, 15.5 average with a stop at the PO to use the machine to send a Priority Mail package, this on the '03's 72" gear (switched Rivs at turnaround point).
OTOH, back in 2008 -- 2005, when I commuted on the '03 Riv fixed across town 15 miles, ~12 mph average by computer was the standard, clock running -- urban blocks and 7 miles uphill, most gradual, 70" gear. (OTOH again, return home was also about 12 mph thanks to headwinds and traffic and fixed gear.) Back when I was a young 40-something, I'd do a slightly longer 16 mile cross-town commute but, instead of back roads, I'd go straight up Central/Route 66, lights a mile apart, and routinely break 60 minutes -- best time >18; liked to pass and stay ahead of the #66 bus; and it wasn't really fun because I felt tired all the time and caught cold easily. ~67" fixed.
For me, it is important to find a good mean between riding slowly enough to stay aerobic and riding energetically for fun. At almost-58 I am getting to the point where I can find and maintain this, particularly for climbing hills while standing. I do really love, love, love at least short steep (=< 1 mile) or gradual longer hills on a fixed gear. Downhill, meh.