Not unlike centuries for many-a-rider, fit or working up to fit.

The key is 1. endurance 2. Nutrition.

That second part has to be right or you'll finish in the red all the time. endurance is a no brainer. The rule of thumb (for me anyway), is to be able to finish 75% of the intended mileage the prior week without incident; the other 25% I make up for with nutrition and just determination as I almost always hurt the last 5 miles or so, just because I push myself.

Another thing, again, YMMV, is to spin more versus mashing ..

All in all, you done great!!!