I do think you are worrying too much about everything, I guess it seems quite daunting to you the tour you are proposing, but fitness comes with riding, each and every day you are on the road will make you more fit, both physically and mentally, when you can ride up a hill or walk the laden bike up a hill, with out huffing and puffing too much then you are ok. We all get at some stage the red mist in our eyes when we have pushed ourselves just that little too hard, but again it is good to just push a little hard sometimes, it helps lift your fitness up a notch as the saying goes “ No Pain no Gain” .
Make sure that you eat, eat, eat keep the blood sugar levels up, because the killer of all is to end up with low blood sugar levels (The Bonk) make sure you drink enough water each day and practise this as well on your training ride. If it really is hot and you are working hard, you can easily drink 3 gallons of liquid during the ride and afterwards as you prepare your body for the next day (Butcombe’s best better or Badgers Screech seem to suit me for a evening top up
I find a heavy bike is not such a problem, on metalled roads, with suitable tyres and inflated to maximum recommended pressures they should roll no problem.
Some years ago, about 3 days before I was due to fly to Singapore to start a tour through Asia, I rode over a pothole in the road on my Reveile Romany tourer, and the seat down tube where it joins the bottom bracket just split away.
My other bike at the time was a Raleigh Mirage MTB made of 504 steel (gas Piping) and very heavy, so it got a quick service, new tyres and was pressed into service, we travelled asome 2500+ miles around Singapore, Malaysia, Thailand and Cambodia no problem at all and that included some very steep hills, good wheels and correctly inflated tyres is the answer and better still good bearings.
And at the end of the days riding or before you ride next day just quickly look over the bike and tighten up any screws which might have loosened, and check that your brakes etc are adjusted, check your tyres to see that no thorns or shards of glass etc are jammed into the rubber or treads which might result in a puncture later in the day and check the tyre pressures.
There Hope that helps and not adds further to an overloaded brain..LOL