Funny you mention your marathon speed. The Cleveland Marathon was this past weekend. My wife's supervisor is 'only' 42, and two years ago he qualified for Boston - he needed a 3:20, and did a 3:18 and change. Last Fall he had a hernia surgery so this year's Boston time was down to 3:40 due to lack of training. Just this past Friday he decided to run the Cleveland on Sunday for the heck of it - no training, etc - and managed a 4:04. Still a LOT better than I could do.

Now on a bike, well, it is relatively flat here out west of Cleveland, so on my commute to work (10.7 miles) I used to try to do the first 8.7 miles in 30 minutes (conveniently there are train tracks at 8.7 miles from each end of the commute) for an average speed of 17.5mph. That includes the five traffic lights. SO my actual pace is a bit quicker than 17.5.

For longer distances - like my 30 mile loop, I try to maintain a pace of around 16-16.5mph -- just under two hours (1:50-1:55) for the 30.1 miles. My century pace is around 16 - six hours of 'seat time', plus added time for breaks.