I'm thinking about running running this scenario for a small group of RPG newbies. I'd like to run it as a one-off for them to get a taste of the fun and not overwhelm them with a lot of rules or stats. I was questioning whether or not running this as a one off would be viable as the time from mereorite impact to when giant ants show up is a considerable amount of lull.

I wanted to hear what some other GMs did for this scenario and if anyone has some suggestions to speed up the time to first contact. Thanks!

In this instance, you may want to just forgo the actual meteor impact in your story and pick up the action closer to when the giant ants actually start showing up. Mentioning the impact in passing could add to the mystery of the scenario (Where are these ants coming from? Did that meteorite he mentioned bring them here, or just mutate our own ants to giant size?)

Yep Chryondave's idea works well. Maybe give your players a bit of background before the game up to the point you want to start running the game from.

Start small and gradually go big.

I started with smaller insects becoming a problem all over the world. Infestation problems, overrun problems, crop destroying etc. As the PCs continued, they ended up bumping into a scientist that had more information about the bug problem. Soon the world was overrun with hives and tunnels everywhere. Its a good way to build onto a story, rather then just saying * HERES A GIANT ANT*

I took the start small and go big in a different direction.

As mentioned before, the meteor impact was days before the actual start of the scenario, with hints that something is going on here and there (missing people who live near the impact site, etc.)

And then, there's the earthquake...

Look for the Invictus Stream on youtube, they've ran this game.