Time on the road is about all that'll make the difference with 3 weeks to go, but temper that time with not pushing it to the point you get an over-use injury or just can't face another hour in the saddle. A broad base is the best preparation now I'd say, (and something that'll give you a good platform to develop from when you have done this ride).
You'll get weary from the distance, even at touring pace, but you'll suffer more if you are uncomfortable, so get your contact points fine tuned over the remaining time.
When you are out getting the time in, set reachable goals - the next town, the next hill, (and come the end of the ride ... the next lamp-post ). Do the same on the actual ride too.
Dunno enough about nutrition for endurance on a bike, but assuming it's similar [enough] to running ... 3 weeks isn't going to be enough time to adapt / train your metabolism to make changes (from burning sugars to burning fats), so maybe stick with what works for now, and sort that out later.