herding stray cattle, land my first ace

Herd does mean to gather cattle together, specifically to direct them somewhere such as new pastures or to the slaughter house. Stray implies nothing as far as quantity. The entire herd may be stray--in this case meaning, essentially, "having wandered where it doesn't belong."

It is difficult to determine the meaning of "land my first ace" in this context. I'm not familiar with this as a common metaphorical expression, but it seems as if "feeling his girlfriend's face" represents some kind of victory.

In baseball, an ace is a very good pitcher. "Landing an ace" would likely refer to a team's management acquiring a very good pitcher. While this would be a victory, it doesn't seem likely as a reference.

In tennis and volleyball, an ace is a ball served to the opponent so well that the opponent can't hit it back. "Landing an ace" would refer to this small victory and seems like a more likely reference than the baseball term. If "feeling his girlfriend's face" is thought to be a step closer towards sexual relations, then a common baseball-derived term is "to get to first base".

Valpower is correct.

You're much hotter than Mrs Shrek,
The first time I saw your face,
I felt the same as when I landed my first ace.

Sorry, last time I don't provide the exact context for I can't find it.

stray does not imply any information about quantity...but the phrase in question was 'stray cattle' and cattle is plural so that does imply something about quantity...