It's not just about leg strength and stamina- a lot of people find the obstacle course very hard due to a lack of upper body strength.
Train bodyweight exercises- burpees, press ups etc.
Train as winter draws in outside- don't stay in the gym or the shock of the cold will be bad!
Run down rivers, turn the shower to freezing, lots of hill sprints and be comfortable running over uneven surfaces.
See you at the finish line!