Maybe instead of going for such long walks in one hit, break it up and take a few more slightly smaller walks?? It sounds like ligament pain. Do you have anything you can wear to help support the weight of your belly while walking?? I get bad ligament pain (and severe hip and pelvis pain) the further along I get in my pregnancy, I have to wear a support belt for my hips and pelvis anyway, but I find it does significantly help ease the ligament pain (even if the other pain still persists). You can get pregnancy support belts from the chemist, or even just some of those tight banding tubes for some additional support.