Last year I did my first double century, and have subsequently done a few centuries. Before that, I was primarily a short distance rider - perhaps I can provide some insight as to my Personal experience.
First, and perhaps the most controversial, I didn't (and still don't) find it necessary to do a lot of long training rides in preparation. I find that if I stay in good shape and make my shorter rides count (i.e. push hard), I'll be just fine strength wise. I recommend at least one long ride before the event, just so you know what it feels like and are not surprised. Otherwise, I generally finish in the top 20% in centuries and rarely train distances greater than 30 miles (but my 30 miles are on hills pushing hard).
The three things that I had to learn about distance riding were 1) moderating my pace and 2) keep fueled and 3) comfort matters.
1) Pace: If you are used to shorter rides, you know that you can get away with pushing hard. For a long ride, you should never push your legs to the point of feeling a burning sensation. In your training, practice climbing hills in an easier gear and spinning faster. Learn ways to push faster without necessarily pushing harder.
2) Fuel: You can get by with no or little food in a short ride, not so with a long one. Teach yourself to eat consistently while riding. Get yourself used to drinking something other than water (energy drink, chocolate milk, etc), and drink often. You want to be used to your favorite riding food before your long ride and have your stomach used to digesting food during a ride. I can't tell you how many riders fail due to an upset stomach when trying new foods for the first time during a ride.
3) Comfort: After a few hours in the saddle, especially on the second day, little things start to matter. Your hands hurt, your butt hurts, your back and shoulder hurts. Get good gloves, high quality padded shorts, and good bar tape - and break all these in before your ride. If you haven't already, get a professional bike fit - it will make a big difference on your knees. Do not underestimate how much mental drain the small aches and pains put on you.
In reality, though, 200km in two days isn't really that far. Lots of people do more in a single day. Get out on your bike, ride hills, do sprints, and ride regularly and you should be just fine.
