I love to ride LD though I have not yet done any IBA trips.
I find my pleasure is to ride for 8 hours a day, multiple days - and depending on the type of routes then I get to 500 miles in that time frame or I don't.
If I were to do the SS I would plan road types that would allow me 500 in 8 hours (Auto Pista in Europe for example) that means I can do 500 miles in 8 hours take an 8 hour break and complete the ride in the final 8 hours. That gives me less than 70 mph for the entire journey and if I exceed that (say average 75) then I get leisurely fuel stops too. Crossing Spain I often do 600+ and even 500 in that time on non-motorway roads - but that is very tiring as you need to be super alert to achieve the speeds necessary to do that. Great roads though. It's amassing how much you sharpen up when you get to a great set of twisties at 400+ mile point after 7 hours in the saddle - great way to end a day LOL.
One of the things I find myself doing to stay alert is calculating where I am in terms of journey percentages. So I know every 50 miles is 10% of my daily target, and I can break it down from there. Some times I am converting kilometers to miles too so it's s pretty active thought process whilst staying safe too,
Getting the calcs down to multiples of 3 miles as a percentage definately keeps you thinking