After I took an extra-long walk yesterday, my muscles are sore.  What's a good way to make them feel better?

Your question is better suited to Diet & Fitness, but some things to do about muscle soreness that may help are:

1-hydrate well (also do so before exercise)2-hot bath (epsom salts added, helps) and/or heating pad and/or warm epsom salt compress3-gentle stretching may help (stretching immediately after your workout, helps too)4-go for a slow, gentle short walk today5-gentle massage or foam roll (if you have one)

Your muscles have micro-damage and your body is trying to repair them.

* Massage

* Warm bath. Some people like to ad epsom salts to a warm bath for sore muscles.

* NSAID (but avoid acetaminophen / paracetamol) 

* Keep moving. Your legs might feel the worst after a rest when you go for a walk. But, if you are able to do a light warm up, followed by swimming or jogging, they should feel better. 

Comment: Although Pearl did give you a non-answer, she does have good advice. When you increase your distance too rapidly, you increase risk of injury. 

probably not to walk so long next time