Düsseldorf has the only marathon, as far as I know, which has unicycles in it. I started twice in that marathon. First in 2008, when I had to quit halfway due to diarrhea. I can assure you that that is not funny on a unicycle. The next year, 2009, I started again and drove it in 2:45, 15 minutes slower than planned.
In 2013 I will start again, same marathon, same unicycle. Not a big 36 inch version, which start in the “unlimited competition”, but a 28 inch version which start in the “standard competition”. My goal will be to beat the 2:30 again. So I know what to train for this winter season.
More info on the RMSV website.