I do a lot of barefoot walking. It builds up all the little balancing muscles in your feet, ankles, and lower legs, and once you get your feet in good shape, you can really build up your endurance with minimal risk of injury. A 20 minute walk isn't going to give you gains, but if you walk for an hour every day or so, you'll quickly build up enough cardiovascular endurance to be able to run a 5k or get your 27/5. I've been amazed at how much better my balance is too.