Quarter of the way to what?

I recently passed the 250,000 lifetime United miles mark. This may seem low, but bear in mind that it's actual miles flow (no bonuses, no promos), and only on United (so no partner carriers, or other airlines). Assuming a rough average of 500MPH, that means I've spent 500 hours of my life on a United plane (which is about 20.8 days). And that's just on United.

Another point about this milestone is that it's a quarter of the way to the coveted "million mile" mark, at which point you receive Premier Exec status for life. So I'm a quarter of the way there. But what then?