I agree with Dragonchilde - muscle soreness is an indicator that you are doing something unfamiliar, rather than the sign of an effective workout.
You say you are a regular walker - well, it's not surprising you don't feel sore, even if you are increasing your speed.
Don't go by how your muscles feel - go by how your heart and lungs feel.
And if you feel you can do more, then do more - add some more speed, and then when you feel you have maxed out your walking speed, then add some hills to your route.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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