Flat Battery

Came to move my car this afternoon to enable the driveway to be jet-washed.  The doors opened - and that was it - nothing....dashboard was blank and the car refused to start.  After a few checks it was obvious the battery was completely flat - a result of being parked up all this year and only started up once which was about 6 weeks ago.  The RAC confirmed all this and warned the battery was not in good condition. The plan now is to run the engine for a hour or so for the next few days and see if the battery comes back to normal.  I have my doubts :-(  :-(