I've been jogging since november last year and can happily go out and run for 25 minutes plus 5 warm up walk and 5 cool down walk, so thought I would give it a go tonight on my bocks and compare the distance/speed.
Jogging on foot I can run 4 miles so average speed is 11.5 mph.
Jogging on bocks on the same route I can run 3 miles and average speed is 8.6mph.
Today is hot and stuffy, plus I was wearing crap trainers, so I will have to try again, and see if I can beat it. It's really hard work and I can't run for very long on the bocks. Trying to make it as smooth as possible but as timtom said every now and then one leg will do something weird and feel that it's then time to slow to a walk, but think this should resolve with pratice.
Would be cool to be able to run for 25 minutes at 25-30 mph. I wouldn't need the car to get to work, would be quicker to bock!
Good luck with the marathon, it must be doable.