Running barefoot does the job for me. And I mean actually barefoot, not with so-called "barefoot" running shoes. Although the barefoot running shoes use the leg muscles in similar way to actual barefoot, you're far more likely to injure your achilles or calf muscle when using the shoes. Going actual barefoot means that the pain in the soles of your feet will tell your body when enough is enough, and you're less likely to tear your tendon/muscle. Still have to be very cautious though, as I've still injured both barefoot. (You're still much more likely to injure your achilles/calf when truly barefoot than with thick soled shoes, but in the end there's a lot less stress on the joints).
Anyhow, it's something you have to work up to VERY slowly to avoid injury, but once you're there, it really works out your legs MUCH more than running with thick soled running shoes. I find that this training makes bushwalking MUCH easier on my legs, and of course it is a good cardio work out.
Currently I'm running about 6km barefoot 2 to 3 times a week on a blue metal bitumen road.
(I tried a short two day bush walk with a full pack while barefoot a few weeks ago as a bit of an experiment. Feet were fine, but calf muscles and hip muscles (?) were VERY sore for a few days afterwards.)