This warning indicates that, most likely, the last backup using this media failed, or was aborted. The last datablock was stored in the IDB as 778, but, when attempting to position the tape, it idi not find the last segment there, instead at 778
This MAY affect your ability to Restore, and, if you have any doubts, you should verify the media, which will sync it up with what is in the IDB
Usually, the length of time that a tape is stored will not affect the ability to Restore, unless the tape is somehow physically damaged. The more likely impediment to REstore is how the data on the tape is protected. If protection expires, the tape will likely need to be exported, and re-imported (which will bring back the data catalog), and re-protected