3x increase in endurance in 3 months?
If you did 65km (40 miles), then after recovery (maybe 1 week) you can probably do that distance again with much less discomfort.  You should then be able to increase your mileage about 10-20% a week, if you get in one max-length ride and one about 1/3rd shorter each week.
Conveniently, I've found that you can usually, if you push yourself, ride about 50% farther than your "accustomed" ride (though you will be dragging at the end).
You do need to eat and drink on a long ride.  I'm a big advocate of this stuff called "food" for that (hopefully provided for you on your ride), though it doesn't hurt to have some sugary snacks.  And did I say DRINK?
90 RPM is probably a hair too fast.  But you do need to keep your cadence up, especially as you begin to tire.  20k/h (12.5mph) is not a terribly fast pace (even allowing for rest stops), so it's important to not burn yourself out trying to maintain a fast pace.


Ultimately, for 180km (112 miles) you will be running on empty near the end -- that distance is challenging.  But by slowly working your endurance up you can probably make it.  DON'T expect to do it with "tricks", though -- the most important thing to do is to just steadily increase your distances.