Well, you're running 1/3 farther on such a short distance, so that is tough to negative split.
Are you running intervals? That's basically what you are teetering into. $0.02

I must confess after my last marathon (Mid-Sept), I've run all of 6 miles in two 3 miles sessions. There are some good reasons for this, but I'm so close on a major milestone for the year, but I'm not even interested.
Cycling has gone way up since then, though.