I had no advance warning either - we were just taxying back to the clubhouse at the end of a lesson when my instructor casually said "I'll hop out and you can do a circuit on your own". My mind thought "who me? - no, wait! I'm not ready!" but somehow I said "OK then" in a croaky dry mouthed sort of way.
I panicked a bit when my instructor radioed ops to tell them that I was going solo, and one of the girls said "OK, I'll put the fire trucks on standby" - standard procedure as I found out afterwards - but a bit of a confidence shaker at the time!!
It was fan flaming tastic!!! - I think I held my breath all the way round. What a rush!