I see what you mean about where you live not having hills, I ocasionally take my bike in the car if I am pushed for time when i want somewhere to do hill training,that's only ok providing you have a car.

To bring your fitness up you could try some interval training which means doing timed efforts with timed recovery, or when out on your rides after a suitable warm up period put in a hard effort to an object in the distance eg, a tree or so many lamposts etc followed by a couple of minutes of steady cycling until your next effort.

By putting in efforts which get your heart rate up will increase fittness but does take quite a bit of self discipline.

Another way is to get a turbo trainer where you can train at home with varied resistances but to do this you have to be very keen as it usually does your head in in no time.

On your trip try not to think about the hills too much its a great route which you should relax and enjoy the countryside, when you do come to a hill just spin up it in your lowest gear using as little effort as you can then enjoy the decents.

Once you get to the 100 mile mark you can look forward to downhill nearly all the time to the end.

Good Luck.