I never grease rear brake cables, just coat 'em with a film of TriFlow. Dribble a little on and spread it around with thumb and fore finger.
At this point I wouldn't worry so much about it. Your braking performance won't be hampered by a slow return and if your wheels are true you won't be slowed much if brake return takes a second instead of a half second. I'd go ahead and wipe excess grease of, however.