I don't know if it is necessary to change the plan. A few years back, I had a series of issues that led to my being in the second-worst shape of my life, but I had the opportunity to ride the 750 mile stretch from my house to my dentist as long as I could do it in a week and a day. I half-followed big blue toe's advice by making the first day shorter than my average, but that was still 90 miles. The ride was mostly along the Pacific Coast, so it was relatively flat (not counting the Lost Coast and the trip from the coast through the Sonoma and Napa valleys), although since it was March I did get slammed by headwinds for half of it.

What could go wrong? I got my first ever saddle sore. To make matters worse, one of my two pairs of cycling shorts was an antique that had a real leather chamois. Having blood/lymph/whatever clot on the leather and then peal off every time I stood up was truly a memorable experience. (I eventually tore off a piece of T-shirt to use as a bandage.) I was whipped at the end of nearly every day and in pain the first hour in the saddle every morning. In short, the trip was awesome and I had a great time. I do that same ride twice every year, usually in five or six days, but it has never been as intense and wondrously challenging as that tour, maybe because that was a rare trip on which I truly did not know if I could complete it.

I'd suggest to the OP to do what is necessary to get into what s/he considers acceptable physical shape. Create some bail-out locations just in case (I recommend something around half-way and another around three-fourths). Then just go do it. If you are the type of person who enjoys a physical and mental challenge, and it sounds like you are, you'll have a blast. If you decide you aren't ready, then find a way to shorten the trip so you have a reference for next time.