I find that it is the fear of the unknowns that produces the apprehension. I am a big believer in taking baby steps, experiencing some of the elements of the tour that you've not tried before and thereby eliminating some of things that can go wrong. Once you are inside "charted territory" from an experience standpoint, you'll eliminate the stress. So, I'd isolate the unknowns (can I ride the distance, will my tent leak, can I cook with this stove, etc., etc.) and see if you can create that experience in isolation. Check that off the list and move on to the next one.
So, right off the bat, it seems to me that setting personal record for the mileage to be ridden isn't what I'd be doing on my first tour. Is there some way to back down on the mileage and eliminate that unknown element of the first tour? If not, can you first try the mileage in a non-tour setting- i.e., just do that mileage and end up back at home? If the mileage works out like this, then incorporating this mileage in a tour won't be so stressful.
Or, if the camping is an unknown, can you do a night in the backyard to see how the gear performs?