All you have done now is delayed being able to do anything.  Leave it alone until it is getting near pollination and spray it with glyphosate.  Use a full label rate and 20 gallons of water and get good coverage.  It won't affect seedling Johnsongrass next year but will cook the roots.  But you have to be patient and wait. If you spray it too early you kill what is there but the family comes to the funeral.  The rhizomes put out new plants in a ring around the one you killed.

The best control I ever got was back in the pre-RR days when I was younger and more ambitious. Field had a huge area Johnsongrass had taken over. I left it alone until the bean leaves turned yellow. I went in and cut off and carried out all the seed heads (yes, that is a lot of work) and using a 2 gallon sprayer applied 2% solution, making sure every leaf got some on it.   100% perfect kill.