My clients suffer from repeat spasms in the tibialis--which is the muscle in the front of the lower leg when they walk for LONG periods. The pain comes quicker when they walk at a faster pace, which tells me the muscle is overly tight. Julie is right...your flexibility--particularly that muscle--is in question.
Here's what I did with some clients...all women by the way, ages 40-52. I had them walk back wards for 2 minutes around a track as a warm up, then they would walk around the track normally for 10 minutes. Then they would switch back to back wards pace. This helps elongate the tibialis muscle actively, while taking the stress off from the repeated dorsiflexion.
Try stretching that area constantly. try this
one.