There's enough debate about endorphins, i.e., about what triggers them and what physical sensation you might experience when you do, that I'd be hestitant to conclude that that's what you're experiencing. There's much talk about a "runner's high" but the scientific evidence is still out on whether that relates to a biological process related to endorphins, some other biological process or soemthing that's "merely" mental. Either way, I feel good after a long or hard ride--for whatever reason.
When you say you feel like you might fall of the bike, do you mean you feel dizzy or do you mean that in a "good" way? Tingly, dizzy, lightheaded, etc., may be a sign that something is wrong.....
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher