Better tip than "just walking" is find a trainer or training group that prepares people for fitness walking (lots of times, local fitness shops will do this for 5K walk/runs, and will have a subgroup focused on walkers.) As with most exercise, proper technique makes a big difference, and even if the human body is built to walk, lots of people (not all, for sure, but a significant fraction of those that would be starting to ramp up from an inactive lifestyle) have developed bad habits and doing it more but still badly won't magically fix them, it will just reinforce them and compound the pain they produce.