You don't become able to walk 50 miles overnight. You need to work your way up, and one way is by sticking to a schedule.
Yup, it is definitely that time of year: when the weather starts to turn beautiful and I begin to get out and walk. It is always such a moment of reality at this time of year when I begin to train for the Challenge Walk. I walked about 3 miles yesterday and can't image building up enough endurance to be able to walk 20 miles in one day by September. It helps me remember that there is a lot of work ahead of you.
But believe me, it is amazing how quickly your body will respond to training and conditioning. The best way to be ready for September is to slowly work your way there. There are some great training schedules; for example, if you start now, you can build your strength slowly with the 18-week training program. (There's also a 12-week training program on page 17 of the training manual)
You can do it! So get on out there, and let me know how the training goes!