How can I increase Marching speed for class apc?

Your APC's will march faster with only vehicles in them. You can also increase this further with a research item under Vehicle Training.

You can also increase speed with certain items crafted in the Parts Factory Building.

I have this same problem also with durability once you research it fully it slows your regeneration back to the begining also your speed. The point is to get you to spend more money. This game is tricky and very sneaky.