A few items to follow up on Valen's re rapid movement along roads:
Mining will often get you farther along a series of roads than attempting to lay railroads due to mining having a longer building queue, though, naturally, building rails as the "last" move would be most beneficial.
"Stored tasks" don't reset between turns. This means that if the settler was "one move" away from completing a task last turn, it will still be one move away this turn.
You can't vary tasks in an attempt to prolong the settler from altering the landscape and thus losing its turn. For example, if building roads would normally take two "assignments" and you've already used "mine" four times, switching back to roads the fifth time will build the roads.
If the settler can't mine or build rails and you don't want to alter the landscape, build a fort.