How to move objects in any shaped path at constant velocity ?

In my 2D Game, military ground units are moving on a ground. But the ground is made up of hills and troughs. What I want is for units to go from left to right or go from right to left on the ground. But the ground is not smooth and it can be damagable by bombs any time. I tried using the physics system so I tried , increasing the friction and lowering the gravity, but units still may stuck on the way. I couldn't get the result I wanted. I also tried sending rays to ground and getting the normal of the ground then convert it to find parallel vector to ground. This way I can manage to move transform by this parallel direction but again ground is not so smooth it still didn't satisfy me. I want objects to stick to ground and move in constant speed whatever the ground shape.

Can you please provide me a way to overcome this issue ?