If the edges of the painted area are distinct you can use the quick selection tool to make your selection.
The edges are not distinct, but rather very soft.
There is a way to insert that painted area into a mask without redoing it. I just cannot find it in the doc.
If you painted onto a separate layer then Cmd/Ctrl-click the layer thumbnail to get an active selection, then add a Vibrance Adjustment Layer. The Vibrance layer will automatically get a mask created from the selection. Hide or delete that painted layer to leave the masked Vibrance layer above the main image.