On a mountain bike I had been working my way to minimize the time it takes me to do a 25-km round-trip.
Then on a road bike I have been trying to do the same for a 40-km round-trip.
My objective on both was to cycle one day, rest the next, then cycle again.
All was well for the 25 km ride. I didn't even need to take a water bottle. Drinking after the ride was enough. For the 40 km ride I was able to maintain the same skip-a-day rhythm as long as I was not pushing myself, though I needed to drink about 1 litre midway, at 20-25C (68-78F). Once I started to race against my preceding time, something went very wrong. After 48 hours I feel too lethargic to hop again on the bike. It doesn't even get better after 72, and barely after 96 hours. I need almost a week to have recovered enough to repeat.
After some searching, the best I've been able to understand is that my 48/72/96-hour lethargy approaching a state of sloth may be caused by glycogen depletion. On multi-day rides, cyclists ought basically gorge themselves before, during, and after with carbs, but I'm not doing multi-day rides.
Hence, I'm guessing, it was a mistake to continue eating the same carb-conscious diet. My calculation for eating steak-and-vegetables, chicken-and-huge-caesar, chickpeas-and-tomatoes at lunch/dinner was that the steak/chicken/chickpeas anyway have large carbohydrate content, in addition to protein, and so I can skip the side of rice/potatoes/pasta, being content that breakfast consists mainly of carbs—bread/oatmeal/..
I was about to experiment with the following classical diet. Eat a (large) plate of pasta 2-3 hours before the ride; eat some more carbs during the ride (2 croissants); then eat protein and vegetables after the ride. The carbs before/during the ride satisfy the needs of the ride, and the protein contributes to muscle repair after the ride.
But this traditional thinking has "largely fallen out of favor as it can also lead to excess water weight and digestive issues."
I realize (I'll soon be patronized and) that 40km is little more than a warm-up for the serious folks. There are several ideas already posted here for longer rides: Gorge on slow carbs post ride, replenish with fast sugars during the ride, snack mid-ride on boiled potatoes + oil + salt; and only one that I could spot for short ones: don't just drink water, drink gatorade. In terms of taste, If find this combination particularly appealing. But rather than experiment on myself over much of the season, I thought I'd ask.
If cycling is seasonal in your area (it's too cold to cycle outside October-to-early-March in my area, and I get bored on a stationary bike after 15 minutes, so I switch to squash and skiing in winter), have you nailed down how to adjust your diet to recover as soon as possible, perhaps even after one day not two?