In some respects it’s ‘just’ a different type of riding, and the hardest part can be the ‘mental’ side of things, keeping going when things get a little tough (eg it’s 2am, you’re soaked, pedalling into a screaming headwind, and still have 100 miles to go!)
I don’t rate myself as a ‘real cyclist, but I’ve struggled through a few of these events, but only one 600k, and that was in Scotland/Northumberland, so perhaps not as hilly as the Bryan Chapman. I too rode the 600k on fixed wheel. I must say that last 200k were pretty tough, and towards the end I was extremely tempted to pack it in and call a taxi!
I’ve never been able to dedicate the time to long distance cycling, so most of my rides have been done on very little preparation. I’ve certainly never got anywhere close to cycling 4,000 miles between January and May! I think once you can do the odd tough shorter ride then you can attempt these, but learning how to get through the rough spells and knowing what to eat/drink/wear/carry can take a while to learn. Of course, the really dedicated riders take it to another level, and make it seem easy while covering mind-boggling distances.