Easier, fun, relaxed, but on pace riding is the way to do an IBA quest ride. The quickest isn't necessarily the easiest, but the longest certainly isn't. Condition. Eat healthy. Prepare. Plan. Because if you do, the training will be harder than actually doing it and you'll enjoy the experience that much more.
I've been helping two other riders gear up for a SS1K in two weeks. Unfortunately, I can only guide them on what has worked for me and I cannot force them to do anything beyond verbal recommendations.
Personally, I've taken this seriously and I do increase my workouts, with an eye on eating healthy, eating lesser portions, reducing high-carb drinks (like beer), proper hydration, and shedding unwanted weight. A fit healthy rider is one that will have something in reserve for adversity. And every IBA ride I've undertaken has had some level of adversity.
One week before each of my three prior IBA SS1K rides, I would click off a 500 miler to acclimate my body and expectations for the real thing the following week. I'd tape to my bike waypoints and time of day along my route, to take the guesswork out of it. And in conjunction with that, as some have said above, I would cease training and increase sleep time 4 - 5 days prior to the ride itself so my body could fully recover.
This year, we couldn't do the week before, so we did two weeks before. We started with four riders and the one who cold-turkeyed this attempt, didn't make it to mile 400 before they were through. No exercise, eating high caffeine energy drinks and snack foods showed in real time the effects of how far off the mark this rider was. He was literally falling asleep, wandering over the double-yellow in turns, and just an overall rolling hazard. We agreed he should stop and he did.
The other two that I've been grooming had a similar diet of beef jerky, PB&J, canned fruit drinks and no exercise. They did better. The first 350 miles seemed to go well for them, but I soon discovered that was an illusion. One had fallen off the pace in a curvy mountainous region and kind of lost their cool. I can't know their mindset, but the fact that we had fallen behind the pace we needed to keep may have stressed them into the realization that this was tougher than they thought. And every 100 miles thereafter we would fall further off pace and their mood soured accordingly.
They didn't prepare, were sore, tired, grumpy, sapped of strength and by the end of our 650 mile ride, had fallen so far off the pace, that even if they were able to maintain the average pace of the previous miles, they would have been bordering on the 24 hour time constraint. To me, this was proof of fitness. The three who didn't prepare would have failed or tried killing themselves to prove something not worth dying for. And to me, that defeats the whole idea of an IBA ride adventure. If it's not going to be a fun rewarding experience, then what is the point?