Often on vacation, you do a lot of walking and climbing that uses muscles that aren't used to working very hard. Take walks, climb stairs, and get those muscles working. If you only walk from your car to the office, you might need to consider walking during your lunch hour. A two mile daily walk will get you in shape quickly. And take the stairs for a month before you travel.
Break Your Shoes In at Home
Don't take a brand new pair of shoes that you've never worn. You are asking for blisters. Try out all your shoes by walking around the block in them. Are they comfortable?
Think Comfort and Layers
Forget the jeans you have to lay on the bed to zip up. Pull out the clothes that move and bend in cheery colors that look good on you. Layers work best for a place you have never visited. You can always peel off or put on a layer as needed. I brought a light weight hoodie to wear under a rain coat with a pair of gloves to the Netherlands. When the weather cooled off, I pulled on the rain coat and gloves. When it got warmer, I simply put my gloves and coat in the back pack.
I wanted to look nice for pictures so I brought pretty shirts and sweaters to wear with my comfy jeans. Wearing light make-up and no jewelry except earrings, I still managed to look nice.
Make a Detailed Itinerary