Below is a week-by-week training plan that will help you reach a goal of riding your bike a distance of 45 miles in one day.

Training Concepts

The main principle of training for a 45 mile ride is to increase your mileage gradually over a number of weeks. By doing it that way, you help avoid injury, burnout and over-fatigue. Plus you will also be able to detect any issues with your body or your bike that you want to discover before the big day.
This ten-week training plan below assumes you are in shape at the start to be able to ride 10 miles comfortably. That's an hour's ride at a very easy 10-12 mph pace.
The best way to learn training, hydration and eating tips is to ride with people who have done it before, but you can certainly do it on your own, or try hiring a cycle coach which will increase your fitness and improve your cycling technique.
During shorter rides, try some hill climbs or steeper routes; this will challenge your body as well as keeping you motivated and interested during your ride.
Also remember to stretch your muscles after every ride, this will prevent your body from developing too much tension and reduce the risk of suffering any injury. Having regular sports massage during the training schedule will encourage relaxation and speed up the recovery between rides.
As you prepare, aim for the targets as laid out in the table below to get you ready. It shows the distance of your longest ride each week (typically on a Saturday or Sunday) plus a cumulative mileage total for the week that you reach with your other riding.

Palace to Palace Training Plan Guide
Week	Length of Long Ride	Total Miles/Week
1	12	25
2	15	30
3	17	36
4	20	40
5	22	45
6	25	50
7	28	55
8	38	65
9	25	35
10	45	45

10 Week Cycle Training Plan