I am a mere mortal. I generally do not race and ride 4-5 hours a week. In preparation for the K100 I upped the amount of riding I did from about three months out. My goal had been to increase the length of my rides by 10 miles per week,hoping to do a 80/90 miler a couple of weeks before the event. It didn't work out like this at all. While I did manage to increase my riding time significantly and for about a month I probably rode about 12hrs a week with two long (40ish miles) offroad rides. I never did get the long rides in. When it got the the race my fitness was OK.
A key element for me was the two 12 hour solo event I had ridden last year. These really helped me in state of mind stakes. I knew that I could ride for the best part of 12 hours. In fact I ended up riding for all of 12hrs 30. These events also helped me practice my eating which is important too. I still got this wrong and it cost me dear.
If I do enter next year I will want to do better so will do something very similar but try to start from a higher fitness base by riding more this winter. I need to work on my speed. I should also move to somewhere with hills.
If I want to get near to 10 hrs I'll need to buy some gears.