Tired of long hikes on Cowle's Mountain

I did 10 miles on Cowle's Mountain this morning with a 16 pound pack (total weight, dressed - 200 pounds), averaging 21:57 per mile.  I was pretty worn out the last couple miles.  I am going to reconsider my plan to do two more long Sunday hikes before the Grand Canyon hike.  I am thinking that two more long hikes would be more likely to leave me tired for the Grand Canyon than to improve my fitness at this point.  I'll think about it some more and then instinctively make the wrong choice.