There's other ways to do it with little or no monetary investment. Swing a maul and split wood, ride a bike.
Running is a really quick way to get in shape. In high school I used to run a lot. I'd walk one lap, then run until I couldn't anymore. Walk and catch my breath, then run until I couldn't anymore. Repeat until I did 8 laps, about 2 miles. Do that everyday for a month and eventually you won't have to stop to catch your breath anymore, you'll be able to keep running as long as you want.
The big downside to running is that it's really brutal on your knees, especially in the long run.