Are you speaking about your fitness NOW, after the trip? Has there been a break since your trip, since you were on the bike everyday? ... or are you still out "on the road" riding now?
If still on the road and feeling weak ... you may have some health issues.
Poor diet? Dunno?
Stamina for long days in the saddle tends to get BETTER as time goes on.
Even in my mid 60's I can ride 10 hours a day ... but it takes a day or 3 to get into the mind set of hammering ON AND ON. Off road going is harder for me now ... super exhausted after a tough day. Day off the bike required.
But once back home and off the bike a week or two, it all goes to Hell, it's back to square one regards fitness and stamina to ride long days.
Join a Gym! Play a Sport! Run! Swim ... have fun! (stress will kill you)
On the road main exercise has always been walking. Walking around the town I'm staying in, often miles and miles of exploring. Stretching is great too. I stretch several times a day at fuel/food stops. Good stretching will extend your riding days and means less aches & pains at day's end.
In my early years I'd stop and do hikes to sites (like Machu Pichu) and in Peru' in the 70's I spent a week in Lima' surfing!
I nearly drowned but was better for it. GREAT EXERCISE!
I swam everyday in Lake Titicacca (freezing
) when staying on Isla del Sol during my 3 months in Bolivia.
In the Caribbean I did a lot of snorkeling. Not super strenuous but fun and good exercise. Body surfing any place there were waves ... GOOD!
Now, fatter, lazier ... mostly just walking now ... and 50 push-ups in the morning.