My experience is that distance becomes less relevant: when you can do 100km, you can do a full day on the bike. At that point you’ve learnt to pace yourself, you’ve learnt to hydrate and eat and it’s just a matter of keep going.
Training MAP (and in effect FTP) will be most useful for LBL. The Mountainous Fondo plan is great for that.
Happy to have a chat via DM if you need further info.
Bragging link here