Posture is definitely a factor. It is so difficult to obtain and maintain what they say is the correct posture.
Feet also play a role. Any problems there will affect your posture as your body compensates.
I found that my chronic head and neck aches are caused by my back muscles being in a mess, knotted, contracted and very angry.
Remedial massage over 4-5 months has shown me that the neck muscles get drawn down and made tight and sore by the contracting trapezius muscle, plus a few others.
So, I'd suggest that you have muscle problems in your back (who doesn't) that are aggravated by walking, leading to the neck pain.
See a remedial or sports masseuse. Expect to have several treatments to gradually tease the muscles out.