Slitherine

I had my titan in the gold bordered hex in south mid part of the map on turn 18, orks failed to kill it on their turn (IIRC they didn't even shoot at it actually). None of my troops came nearer than the river west of the victory hex.

I'm assuming victory hex is the gold-highlighted hex in south center of map?

I was given "defeat" message. What did I do wrong? Do you need your titan in prior to turn 18? How do I move past this scenario?

Yup that is correct, you do need to move to that location before the turn limit expires. And checking, there is only a 12 turn limit on this scenario, not 18!

Ideally, your standard forces clear a path so the Titan can get there in time, the Titan trying to fight its way there on its own may be why you are running out of time on this scenario.

Sorry is 12 not 18, just checked - last turn is what I meant.

I walked into it on last turn. So when scenario ended I was in the hex. Is that not good enough?

For anyone else who had a problem here, I restarted a few times and still failed to have victory. Then I restarted again and and immediately just started walking the titan southwest toward the victory hex. I destroyed units and moved through ZOC's slowly as needed. I reached the victory hex around turn 9 I think and went into it. A turn or 2 later I was given the victory message. I guess the trick is just get in there quickly don't wait for turn 12.