Starter won't disengage

[jfnar]

I went to my parents' house to check on them at lunch and when I was leaving I started my truck but the starter wouldn't disengage. I tried several times and it eventually seemed to disengage normally and I started down the road. Got only a few yards before I heard the starter whirring. I had to back it into their yard and borrow their truck to go back to work. Any suggestions on the cause of this? Never happened before. 68 gmc with 292 straight 6. Any help will be appreciated.