Footwear: Plan for the terrain to decide if you need your regular walking shoes (pavement and roads) or trail shoes/boots (natural trails). For after the walk, a second pair of comfortable shoes or sandals will give your feet a breather. Top Picks for Cushioned Shoes | Top Picks for Trail Shoes
Socks: Take a minimum of three pairs of socks to allow for changing, washing, and drying time. Top Picks for Walking Socks
Underwear: Again, a minimum of three changes to allow for washing and drying. Women should also consider three sports bras.
Pants: Choose a lightweight weave of cotton or cotton/poly that dry quickly. A single pair is fine for warm climates, two pairs if traveling to moderate or cool climates or plan to wear the pants while walking. Top Picks for Walking Pants
Shirts: Coolmax shirts, both long sleeved and short sleeved dry quickly when washed. For a walking vacation, take along two short sleeved and one long sleeved. Top Picks for Summer Walking Shirts
Jacket: Take a windproof water-resistant jacket with rain hood. Top Picks for Waterproof Jackets
Sweater or microfleece vest: This will provide the insulating layer when traveling to moderate or cooler climates.
Hat: Protect your head from sunburn, or keep your body from cooling down with the right headgear.
Sunglasses: I never leave home without them.
Sunscreen: A walker's essential to prevent sunburn and skin cancer.
Bug repellent: Know where you are going and take along bug stuff if needed.
Maps: Know where you are going, or where you are supposed to be.
Pack: Choose a backpack or hip pack to match the distance you will be walking and your needs along the way. Try it out before your trip on walks of similar length to make sure it works well for you.