If you don't regularly do 16km rides it would be really heavy to do one out of the blue. Every time I've biked that much it was a true voyage :') and I was young that time.

And I'm from Holland and used to bike every day when I lived there. I wasn't exactly fit (we don't have that American reverence for sports or fitness in Holland) but not terrible either.

Now I don't own a bike at all but I walk a lot.