I bet you are a beginner bicycle lover. You like to make some challenges with your friends but here is a really big one: 250 KM in just on day! This is possible and really simple even if you are a beginner. With the right train soon you will be ready for the big ride. If you are an absolute beginner who doesn´t ride for more than a year, you need just 3 months of training.
In the first month, you need to start slowly. Get a speedometer and never ride at more than 20 KM/H. Your entire body needs to adapt to this new sport. Don´t push too much right now. In the first week train 30 km per day every day. Rest Sunday. In the second week ride 60 km per day. You should ride about 4 hours per day. For the Big Ride you need to ride with an average speed of 20 km/h. If you have time train 90 km per day, but if you can´t just train the 60 km and try to increase your average speed. The goal is ridding those 60 km at an average of 25 km/h. That´s all you need to do for the 3 months. Sometimes people take less time.
For the big ride get ready to sit for more than 12 hours on the bicycle. Drink plenty of water but little by little. Drink 50 cl per hour. Every hour rest 5 minutes and eat a snack, every hour! You will need a lot of energy.
Listen to music while you ride. Boredom will be you biggest enemy. You will not be very tired if you make an average of 20 km/h but your back and your legs will hurt because of the bicycle position.
That´s it! Good luck.