The majority of walking that you do in a day at Disney is nothing comparable to walking a marathon. At Disney you might walk “spurts” that end up equaling 13 miles, but that is going to be over a 12 hour plus day. You will have water breaks, coffee breaks, ice cream breaks, lunch breaks, ride breaks. There is no need to go all out in your “training” ( unless you need a motivation to do so) but realistically the longest stretch you would need to be ready for might be a straight mile or two before having something to do in between. I would just make sure you are eating healthy, taking care of yourself and can walk 2-3 miles comfortably. Odds are you won’t even go that far without stopping. Walking 13 miles in 13 hours only comes down to 1 mile an hour, mist everyone can accomplish that easily ( not including those with physical or medical limitations)
As far as knee pain, ibuprofen in the am, mid day and before bed will help the most. You can also always take a “day off” from the parks and a few long soaks ( no longer than 15 minutes at a time) in the hot tub or bath