Your internal reserves (glycogen) run out somewhere around 50-70 miles, so you will need to refuel/eat every 15-20 miles from 40miles onward.
Fluid intake will also be even more important, especially in temps above 80F or so. Figure on at least 1 oz per mile, and even that may not be
enough. If the area is very hilly, this will take a lot more out of you than flatter terrain and wind can be a big factor as can whether you bike
through a forested area versus wide open terrain. Once fatigue sets in, some people lose their appetite and don't drink as much, which accelerates
the decay of ability. It helps to do progressively longer rides, bumping up 30-40% each increment to see how you respond to the stress. A saddle/
shorts combo that is fine for 60, may be a mess at 140 miles. Trainer riding in an air conditioned environment is not going to be enough.