I have done century rides where after I'm ready for more and can stay up all night and I have done 20 milers where afterwards I'm exhausted. So many factors to consider and after you gain some experience you know how certain things affect your ride:

Nutrition before during and after a ride: What you eat even days before a ride can affect your performance. What you use during the ride to keep your electrolytes and stamina up and keep you from getting dehydrated is very important. After a ride you should replenish your body with protein and good carbs. Something like V8 juice is good.

Hydration: Nothing make me more exhausted than poor hydration. If its hot out I tend to not do as well. Cool weather I can ride forever. I can climb like a billy goat. Hot out, I am a slug. Some people are just the opposite.

Too little or too much training: Of course too little training will affect performance but too much will as well. You might feel great the first coulpe of rides but then it all catches up with you. I always feel best the first ride after a slight layoff.

Other things like road conditions can be a factor. Was it breezy one day and nto the next? Are the roads slick, were there alot of hills or was the ride flat.

No one expect every ride to be the same. I never know what to expect. That's the thrill of getting out and giving it go. I have learned,however, after my ride to not come right home and veg. I get home, shower, and then start doing all my chores. I find if I engage myself I start to feel better. If I immediaitely hit the couch then I'm a slug the entire day.