Having only run a couple of marathons before, this was my first off road marathon and definitely the most hilly I have tackled - but I am now thoroughly converted! Nothing beats stunning views, muddy legs and great descents. 
If you want to run a very scientific race, with carefully calculated split times and completely consistent pace this may not be the race for you, but if you want the satisfcation of completing a difficult challenge and overcoming whatever obstacles come your way on the day, you'll love the Clarendon. The race day had great support along the course and was really well marshalled throughout. I was worried about drinking enough during the race, but in the end I didn't even use every water station.
I am fortunate to live locally, allowing me to train on the course a lot in the run up to the race. I'm really pleased I did this, and I encourage you to train on the course (or a similar environment closer to home) for the following reasons:
- The route is not flat - but there is plenty descent to enjoy as well! Rising out of Salisbury, descending into Broughton, rising out of King Sombourne and descending into West Wood is all significant height change but this does allow you to see some views that are so stunning that they momentarily distract you from the pain in your legs! Learning to increase my stride downhill helped save time, and running some of the tougher hills beforehand meant that I knew the hill had a top, and wasn't completely endless.
- The route is definitely off road. Although I was pleasantly surprised by the quality of a lot of the tracks there are definitely sections of muddy, rutted and root covered track which require a little more concentration and careful foot placement. Again, I felt like after a couple of months of training I "got my eye in" and felt a lot more comfortable on this terrain.
All in all, the Clarendon Marathon is the most enjoyable and well organised race I have entered and I look forward to entering again in the future.