This workout is a tough conditioning challenge so be prepared to suck at it the first few times you give it a go.
Find a hill of approximately 50m. Put a sandbag that is around 25% of your bodyweight across your shoulders and run to the top of the hill as fast as you can. Walk back recovery to the start (or run if you're feeling brave) and repeat.
Aim to complete as many hill sprints as possible in 20 minutes.