It's funny, I got stuck on a bridge for a while on the commute home tonight and thought about having to hoof it if it and the others between work and home were somehow out of service. Of course, my trek would be on relatively level ground, and only about 60 kms, so not nearly the scale of yours.

I'd start by thinking of what you're going to wear.

First off, I'd want REALLY good already broken-in footwear, a REALLY good pair of already broken-in back-ups, and a few pairs of REALLY good socks. Your feet will be your salvation. Take REALLY good care of them! Blister care and moisture protection would be right up there too, as would a repair kit for my footwear, including the socks.

Your clothing will be essential too. It's your first line of protection against the elements. Plan for all possible weather and bring back-ups, including underwear. I'd pack multi-tasking (weather-wise) clothes where possible, to lighten your load. Plan a layering system, and consider "wickability" to help you stay warm and "soakability" to help keep you cool. I like wool and synthetics personally, but I am partial to loose fitting cotton when it gets hot.

Drifting a bit into shelter, a wool poncho retains warmth when wet, and helps protect you from the wind. It's easy to wear and can be used as a blanket, it's fire and heat resistant, and it can make a roof for a lean-to if it's not too wet.