I've mentioned before that I was born with stiff joints and stiff muscles. My condition is known as the "squeaky door" syndrome. Just as a door with very rusty hinges is hard to get moving but once it moves it moves more freely, my muscles are stiff when I first get up but feel better as I move around. I got up a couple of times during the night to get a drink of water, and my feet were very stiff when I walked into the bathroom. They felt better on my way back to bed. They aren't used to the 9 miles that I ran last night. After a couple of weeks of that distance, my feet will feel better. This is nothing to worry about -- just the way my body is built.
This squeaky door problem is the reason I started running many years ago. I've always been active and done a lot of walking and hiking. But, after finishing college, I bought a car and began driving instead of walking as much. Over a period of a couple of years, the muscles in my feet deteriorated and starting hurting when I was on my feet a lot. After a day of yard work, while I was in Phoenix, my feet would hurt for two days. The doctor told me to do anything I wanted that would strengthen the muscles in my feet. I began running. My feet are still stiff after I'm on my feet a lot, but I no longer have the pain in my feet. I also use a homeopathic remedy called Rhus Tox when my feet are stiff. That remedy really helps a lot. It is made from diluted poison ivy.