There is a wide range in human ability, and only you know what is reasonable for you, and how much you are willing to suffer.
That said, I am 59. bike commute every day and ride centuries (and sometimes a double) and do supported touring with my wife. I hit about 7000 miles a year.
My reason for for bring this up is this: I am not an athlete. I wish I was, was I never have been. No high school sports, club sports or anything.
My thinking is that if I, non-athlete can do that much, you can do your trip if you are willing to hurt just a little.
Then again, if there is no stop watch running, you may not even need to suffer!

Sounds like a fun trip.