Agree with Rando. It's just pacing. You can attempt to exhaust yourself just as thoroughly on a 100k, 200k, 300k, 400k, etc. You just pace it so you last longer. The simplest way to do this is to put an upper limit on your HR right from the start. Period. No HR over that. Adjust that limit to the length of ride by experimentation. There are other systems, but that's the simplest and most foolproof.

So you could try using the climbing HR you could achieve at the end of a hard 100k as your HR limit for 200k. For every ride over 200k, use the ending HR at the previous distance as the starting HR for the longer distance. At least that's something to start with.

Oh, sorry. You asked about mental attitude. For me it's just the idea that yes, I can ride in pain for a very long time. A very, very long time. No permanent damage. So it's just a matter of regulating the intensity of that pain so I reach the finish still pedaling and not walking. Like LA, some of us do it for the pain. You just thought you were exhausted. Heh. There's a reason it's called The Dark Side.