Cycling Vietnam

I'm going long distance cycling this summer (about 1200 miles) along the coast of Vietnam.
What should I prepare the muscles to get back in shape? There are some hills but not too much and not too large. I will try to cover the entire distance of about 20 to 25 days.
What should I do to prepare?

quadriceps and calves especially. In other words your upper and lower legs. In addition, the hamstrings

Your calves for most. And all the muscles of your legs. You must also run and walk because they are different muscles of the leg.

Train the muscles of your bike. To do this, for riding. If the weather does not cooperate, where you live, get a trainer inside.

There is no substitute for mileage.

You train for the bike while cycling. It is not only to strengthen your muscles, you also get used to sitting on a bicycle seat, and be in the position you take on your bike. Just go out and ride a bicycle slowly increasing distance each day. Training for recreational cycling is not complicated, just go out and ride a bike.