First Long walk since knee surgery

I am planning on doing a long walk tomorrow, 13 miles, by doing 2 6.5 miles back to back. The first one will be with my usual Tuesday walking group and when I finish that I plan to do it again. I am not going to mention doing it to my group of friends as they will worry about me being along. I'll finish our walk, let them leave, then repeat it. It won't be nearly as much fun the second time, I am sure, but I need to know where I really stand in my training. If I am dead tired near the end of the first walk I will shorten the second one by leaving my group early and going up to the House of Dreams. That will make me have 10 or 11 miles instead of 13 but that will be fine. In fact, that is what I should probable do.