Train cycling legs for a long walking trek Cycling is my sport. I'm training for a multi-day ride (four days). Each day will we be hard riding but not too long. The week before the event however I'm going on a four day trek with a 20kg backpack. 

I'm happy to lose some form for the event and have a great time treking with friends. However the last time I did a trek like that my legs were toast for a whole week afterwards. The uphills were a breeze but it was the 20km downhill that killed me. My legs are made for pushing pedals, not going downhill! 

Any advice on the minimum amount of training to survive the walk and be good to go for the cycling event?
I'm with you on this. My summers often include a mix of cycling and hiking/backpacking, where cycling is the dominant activity.

Two things that have helped me. The major one is to use trekking poles. I discovered trekking poles one summer after breezing through some difficult rides, and then being devastated on a not too serious hike. The poles significantly reduce the leg impact, especially on the descents. The second thing is to cross-train between hiking and cycling; but that depends on your personal time constraints and goals.