The group walked for six hours a day, battling altitudes that made her heart race and in temperatures so cold her water bottle froze.
"The coldest night we had was [about] minus 18C outside [and about] minus 10C in the small accommodation rooms," she recounted.
"After 5pm there wasn't anywhere you'd want to be other than in range of the furnace in the tea house common room defrosting your fingers."
Girls to benefit from One Girl - and the fundraising efforts of its supporters including Miss Cassidy - include orphans, young mothers and others living in severe poverty.
Barriers to education in Sierra Leone include early marriage, pregnancy, poverty and the Ebola epidemic.
On the web: my.onegirl.org.au