That fella looks like a beast in the gym. Is that you?
I experience this rucking as do most I know; the front of my lower legs feel like they are on fire after about 1/2-1 mile while rucking. When I feel it burning I will usually jog a bit to calm it down. Then you start getting what we call clown feet, where it starts to feel like you almost cannot feel your feet hitting the ground. After 3-4 miles my legs “calm down” or loosen up and it feels like I can just take off and increase speed considerably.