Seattle Forrest

Sure, but it's more than that.

You heard you should work out for at least a half hour, as hard as you can, so that you can barely have a conversation above the gasping for air. To the point that you'll have trouble standing up when it's done. Why do you think people don't just do a workout like that every day, maybe once in the morning and once in the evening? Because they can't.

Exercise stresses your body in different ways. For example when you go all out, you're using lots of glycogen to make your muscles work; it's not the best thing for them, and it's also the only fuel your brain can use. When you run out, it takes a while to produce more. That's what it means to "bonk" or "hit the wall." It's also why intense rides make you really hungry, your body wants what it needs to replenish your glycogen stores. Longer but less intense rides can make your legs do the same amount of work, but more slowly so you're mostly burning fat instead.

After exercising, you don't have as much energy, and your body needs time to repair the damage you've done, and to get stronger as a result of it. The more intense the exercise, the more time you need for recovery.

Long, moderate intensity rides will also make your legs and your heart and your lungs stronger. They'll do it at a lower physiological cost than just going all out.