I don't know of a faster way as this setting isn't available in bulk. IMO, this would be kind of a dangerous thing to allow bulk editing with.
That being said, reports have rights independent of the dashboards they appear on. So you can add a "runAsUser:name" column to any reports report and see what the Run As permissions are. Then change each accordingly.
displayname=Run As User
linkedname=runAsUser
namekey=view.relatedcolumn
namekeyargkey.0=runAsUser
namekeyargkey.1=name
querysort=runAsUser:name
valuefield=runAsUser:name
valueformat=HTML