running, biking, hiking with weight, skipping rope, jogging or running with a weighted vest, sledge hammer or maul work with a weighted vest. Any kind of resistance training or conditioning work that includes a heavy cardio/anaerobic mixture.
a lot of general physical preparedness type exercises. endurance will slowly but surely improve, some months you will see unexpected leaps depending on how you mix your workouts.
picking a loop to bike and when you get to rough spots putting the bike on your back and running, see how many times you can do a loop in an hour or two. Always try and improve the time/how many loops you can complete. I've seen good leaps in endurance while doing this, esp with a weighted vest.
20-30 rep squats at a weight you might only regularly do 10reps with. Try that out with proper safety precautions.