Yes, it's every hour. But on one of the attempts the driver raced for 15 hours straight and then used his accumulated 10minute breaks all together. I fail to see how on earth that constitutes a 'marathon' as the record title suggests.
Also, the latest holders were using Grid, if I remember correctly, according to one article. Somebody in this thread said it wasn't even possible to do one continuous 'marathon' race on that title so they clearly didn't run just one race.
This weekend I will (providing I have time) write to Guinness and try to find out more about what these other parties did and whether we can submit our event under a new title making it more fitting to a continuous race.