Hello PB ,
Greetings from North Cheshire .
Advice get yourself a good comfortable pair of boots . Start walking ASAP .
Upping your milage accordingly .
You have just over 3 months to get fit for the distance 26 miles ,which is achievable .
You must focus on your goal . completing and raising money for a charitable cause is all the Inspiration you need .
PS . What is the Charity you are walking for ? Also how far have you walked distance wise
Anyway Best Wish's ,and I'm sure there ate lots of good people on the forum who will give lots of advice and encouragement .