Bowwild,
I've been doing "aerobic archery" for a little more than a year now. Lost fifty pounds! You're right, lots more fun than just walking, and a lot easier on arrows!
I use 50 lb feed sacks stuffed with plastic. I have 12 of them scattered across about 20 steep acres of woods. So far, I've only had a few of them wear out. When they start to wear out, I just carry a new feed sack along and stuff the contents of the old bag into into the new one. I use zip ties to close the ends of the bags.
The feed sacks are 40 cents each at the local feed store. You can draw any target you want on the plastic feed sacks with a permanent marker.