Your fitness level really affects how much activity you find tiring. I remember back when I was a teenager working at a grocery store I freaked out because I saw a woman dripping with sweat and panting (I was worried it was a heart attack or other medical emergency) and she explained that she's fine, she just decided to walk her shopping instead of getting a motorized cart today.

Or for a more personal anecdote, I got on my bike for the first time in a decade early last spring and barely made it one block on the bike before being at the nearly-vomitimg-from-over-excersion point then by biking every day I got up to biking 8 miles a ride by the end of fall