Did it in 2010 and 2011. In the dry it is tough but ok, in the wet it was brutal on body and bike – As above I think 2011 K100 85miles marks the lowest I have ever felt on a bike but I feel good having done it*.
It is not that technical and there is a lot of easy fireroad riding. Everything gets hard towards the end as you are exhausted. I didn’t have enough left in either year to really enjoy the descents in the last quarter – besides I was using them to rest. There are some really good sections in there but it is a 100mile ride not a fun action packed blast.
If you are planning training for it then I would sugget you need to get out on your bike for plenty of 7hour+ rides.
You need to get comfortable. Work out if you want bottles or a camelback. I prefered loading up my bike rather than my back. You need clothing that will work in all weathers and is comfortable and doesn’t chaffe. Your bike needs to be in excellent working order because any small fault will be a race ender if it is wet. Food and drink – what can you eat for 10hours + while riding. Gels may be fine for a 3 hour race but not after 10.
*You can get fit enough – but you also need to get motivated and work out what it is that is going to get you to the finish. In 2011 it was as much a mental challenge as a physical one for me.