Make sure you attach the top of the runners to something real, with real metal clips or brackets.
The load of two folks walking up the steps can be amazing. Not quite the same as driving a car up the steps, but almost the same as parking one on them.
The 2X12 steps should set level on the runners. Each gets two screws into the runners.
In our area we need to have the risers to stop the snakes from biting our toes. But, really they give a great deal of streght to the step and stop all flexing.
I even put a little stain on the risers to make the new steps better match the old deck.
Makes me dizzy just to look at this. Be prepared to pay a lot for these and do not even think of doing this yourself.