You are only doing short segments of the walk, not the whole distance! If you look at the daily itenary it doesn't look too strenuous:-
Day 2 - 3.2 miles – 90 minutes
Day 3 - 3.1 miles – 90 minutes
Day 4 - 3.1 miles – 90 minutes
Day 5 - 3.7 miles – 2 hours
Day 6 - 3.1 miles – 90 minutes
Day 7 - 1.9 miles – 2 hours
A good reason to gradually increase your current daily walks in advance?