Ideally, you would train in a similar fashion to how the event will be. However, this is often impractical, especially for longer / harder events.
So, I think the answer is...work out how many hours a week you have to train, and then work backwards from that, with less time to train meaning doing more high intensity sessions each week, and vice versa.
For example...
If you only have a few hours a week to train, e.g. less than 8 hours, then high intensity training (like the TCC programs) is probably the best way to improve your fitness given your time constraints, even for long bike rides.
If you have more hours to train each week, then you would probably benefit from a mixture of longer endurance rides (which I expect a marathon MB ride to be) and just one or two high intensity sessions each week. The longer endurance rides will help your body become more efficient when pedalling for several hours, whilst the high intensity sessions (like those in the TCC) will help you increase your lactate threshold and VO2 Max which will enable you to cruise at faster speeds and also get up steep climbs more easily.
P.S. I know a guy who rode 300 miles (around 480 kilometres) in 12 hours on his TT bike by training 5 days a week for only 1.5 - 2 hours for each session, usually with 3 high intensity workouts and 2 medium intensity workouts each week...so if time is short, doing well in longer rides can still be done