Oci-One Kanubi wrote:

I was kinda sore by the end of day two; then I remembered that this was
just how I felt when I was a beginning paddler. At the time I thought
it was inexperience and un-conditioned muscles. Now I realize that,
when I was beginning, all the trips were led by old-schoolers in those
big long boats, whereas I have always paddled shorter whitewater boats.
So now I think those old guys weren't so much tougher than I after all!

Also it's possible your paddling is more efficient now, both in terms of
stroke selection and execution.

Something similar is often noted on on Grand Canyon trips: a novice
takes the oars, and gets really winded and tired just making lines.
Then an experienced rower takes over, and guides the raft effortlessly.