Its hilly on a lot of track. I ran the flats and downs, but marched up the hills. I found doing this I did not get over taken once and allowed a steady pace. You could run it all if you had the life left in you. The last hill is steep and comes down a wet grass verge which can be a bit precarious. Once the run is done, most I saw got cramp sitting on the kayak, this was best avoided by sitting cross legged - surprisingly works well.
I did it in a day last year and had same question. I used a hard tail mtb with thin hybrid tyres and tri bars. I kept with road bikes on up and downhills, but they fly past you on the flats. There are a fair few miles of this. the off road was mainly fire track and a careful road bike would be fine. The only mud section was less than half a mile, while I flew past them, they caught me on the next road. Personally cross bike best, but I would consider a road bike with slightly gripper tyres if I did again. Recon i would knock a good half hour off my 12 hour time. Hope that helps.