Since you’ve done 70 that can help with the mental part. Having a training plan has always worked best for me. I’d work out a training program designed for riding several consecutive days. A simple one could be building a base with steady high spin easy riding during the week, then start adding strength building mixing short sustained effort sprints and hills balanced with easy rides into the weekly plan and then decreasing the strength building and adding longer rides, up to say 75% of your final goal, once or twice a week to build endurance, help condition your muscles and get your body used to extended periods on a bike and finally ride consecutive day rides of 50 or so miles over the same kind of terrain as your June ride and a couple of weeks before. Decrease training to rest your muscles the week before but keep your calorie intake up to build your fuel. Keeping your training rides in the weekly 10%- increase area should help you design the timeline of your plan from where you are now physically. Rest and recovery time is important to include in your plan. Some body weight exercises to build core strength will help greatly too. Eat well and use more complex carbs to keep up with increasing demands for energy. Try different foods and drinks during your training rides to find out what works best for you and stay with that during the June ride.

Just enjoy the challenge, the June ride no matter how long it takes you and meeting kindred souls of the charity on the ride!!! It may even lead to joining Audax UK