LeeG

It's clear you understand the need for a rest day but seem unclear on the reason. You can have a rest day every few days where you make it an easy low mileage day. You can have a rest day where it's a long downwind ride most of the day and you take it easy never exerting yourself, you can have a rest day every five days where you hardly ride the bike, you can have a rest day where you camp in the same spot two nights and stay off the bike. You aren't resting because of a calorie deficit you're resting because the cumulative exertion is beyond your normal day to day experience and your body needs rest to repair and recover.

Some folks have a base conditioning, heart, lungs, gut, butt, back, hands, etc. that they can ride one 50mile day after another until the seasons change. If that person tries consecutive 75mile days they'll probably need recovery days as the exertion is more than they are accustomed to. The rest isn't because they aren't taking in enough food it's because the exertion is more than normal and it takes time to recover. With adequate recovery over weeks, months and a season or two they could have a higher base fitness and put in longer consecutive mile days.

Or if that same person who can ride 50milers back to back until the cows came home tried to increase their effort and speed they'd have to recover from that. Instead of a 4.5hr 50miler they tried a 4hr 50miler where they jammed up every hill and pushed harder on the flats. That increase in exertion would require recovery days just as long mile days. If that 50mile in 4.5hrs person wants to make it in 3.5hrs they will have to go substantially harder up hills which is very easy to fuel but there's a limit how much hp they can put out the muscle, joint and connective tissue strain is something that may take days to recover from. It wasn't the calorie exertion but the strain. It doesn't take much extra calories of food to put out a few hundred extra calories of energy but the issue becomes the strain on your muscles and connective tissue to transmit that greater power. Like weight lifting it takes time to develop the greater strength through cycles of stress and recovery. Simply eating more food won't enable you continue a higher level of exertion. Milkshakes don't make a weight lifter strong, it's training and recovery.

Recovery/rest, whether it's minutes after a sprint, taking a rest after a mtn climb, or a day off after consecutive days of hard riding isn't an issue of calorie intake it's because your body has limits in energy output and when you approach or exceed those limits you need to recover. A milkshake won't fix a sore knee, chafed skin, tired exhausted muscles, irritated tendons, tired knees. etc.