You can usually pick up an old Nordic Track XC ski machine pretty inexpensive. By varying the tension and/or incline you can keep it to a walking pace and still get your heart rate up. The down sides are: 1) coordination - it does require a bit of technique, 2) knowing the effort - unless you can find one with a working computer you'll have a harder time tracking workout progress (miles, feet elevation, etc.). But, for $100 or so you can have a machine that will get you workout that will get you in good cardio shape for hunting. You can even put a loaded pack on for your workouts.
But, whatever you do - get a decent heart rate monitor. It will help you know when to back off. Most people push too hard in their cardio efforts and get into an anaerobic zone.