Five miles is about 8.05 kilometres (exactly 8.04672 km, since one mile is defined as 1.609344 km). So a 5-mile walk is roughly 8 km, a little more than a 8 km route.
A quick way to convert in your head: multiply miles by 1.6. For 5 miles that's 8 km. If you want to go the other way, multiply km by 0.62, so 10 km is about 6.2 miles.
If the organizers post a route map, check whether the distance they list is the official measured course. Charity walks often round up or use a marked loop, and a GPS watch can read a few percent differently from the marked distance. Good luck on the walk.
