I just did a 100k road race, perfectly flat and due to late notice couldn't really change my training. I basically trained as normal, 50-60 mile weeks, did the High Peak Marathon 3 weeks earlier, a few 5-8 hr runs in the hills and weekly 10-13 mile road runs and felt OK.
I finished in 7:49, but my second 50k was 4:05, compared to 3:45 for the first 50k. 10k splits of 43-49 mins. Where I felt I lacked was conditioning in the legs for running on the roads, by 70k my quads and feet were getting sore. Fitness/stamina wise I felt strong, strode out the final k to gain a place.
I'd say I'm in a worse state post race than most who raced due to my legs just not being conditioned to the terrain, although on the day performance
I held me own (11th in a strong international field).
I was pleasing to know the old time on the feet training worked, but with more specific training I think it could have been a more enjoyable day out..well as much as 50 x 2 k loops can be..