To do 25 miles, no problem. But to do 4x that amount, as a relatively untrained athlete, you are going to have to take in a lot more glycogen while also exercising. Otherwise, your body ‘bonks’ when your available glycogen stores have been consumed.

Fat is also an energy store but not a great immediate-term store of glycogen. Fat has to go through a number of metabolic processes before it’s converted to available glycogen.

Covering 100 miles also introduces things you may not have appreciated in a shorter, more controlled ride - wind, climbing, heat; mechanical issues like flats, chains or derailleurs; navigating traffic, cyclists, pedestrians, terrain, crashes; back & shoulder aches, numbness, saddle sores, chaffing, gastrointestinal issues, etc.

Part of endurance sport isn’t just training your muscles - or even your mind - but also your gut to be able process the carbs you’ll need to make a huge effort.