Here is a program you can use to graduate to rides of up to 100 miles, better known as a century. This training calls for hard efforts, plus rides with groups. You need to become comfortable around other riders because most events have large fields. You'll also need to experience riding in a pace line or a group; a great way to conserve energy without losing speed.
If you follow this schedule for about eight weeks, adding no more than five to 10 percent distance/time increase each week; you should be able to complete a century.
Do not attempt this program unless you have been riding consistently for several months at a modest effort.
Monday: Day off to rest and recover from weekend rides. Use this day for weight training if it's part of your program, but do upper body only no leg exercises.
Tuesday: 75 to 120 minutes in Zone 2, except for 30 to 45 minutes of sporadic high zone 3 efforts in the form of short time trials, hill jams, pushing into a headwind, and so on.
Wednesday: Day off from riding, or up to 60 minutes in Zone 1. Weight-train as on Monday.
Thursday: 60 to 90 minutes in Zone 2, except for one 30 to 45-minute interval in the low end of zone 3.
Friday: Day off from riding, or up to 60 minutes in Zone 1. Weight train as on Monday.
Saturday: Two to three hours with a small group on flat terrain. A good way to complete this ride is by finding a fast paceline or group ride. Ideally, you will be at the higher limit of Zone 3 much of the time.
Sunday: Two hours in the hills, climbing in high limit of Zone 2 with short periods (2 to 3 minutes) in high Zone 3. Between climbs, ride in the lower portion of Zone 2.