Discs do undergo degenerative changes with age and may be difficult to pin point it to the 'sitting at desk for 8 hours'. However sometimes abnormal postures while sitting at desk pose excessive strain on the vertebrae which can and should be avoided. The tear takes some time to heal and you can go back to work. once it has torn there remains a possibility of it recurring irrespective of the job that you do. Learn some physiotherapy
, avoid unhealthy postures and everything should be fine.