I guess that moving on non road/river terrain cost 2 movements point per title.

You need at least 1 point in order to be able to move. And your initial movement speed it set to 1. And movement cost associated with the tile determines if you can make another move

It depends of the rule. If the rule was: you can only move into a tile if you have at least the movement cost of the tile.

But it isn't the case. And it is lucky since all units have a default movement point of 1 and nearly all tiles have a movement cost of at least 2.