Something that helped me: Try to hit almost every box that's in the way. The gap between buildings is entirely dependent on your current speed so the slower you're going the closer they are and you have more time to react as well. Be careful not to hit too many boxes when they're right next to each other as you can slow down too much from multiple hits for the already determined gap to the next building. Doing this, my best so far is 8354m.
I started working on a new version last night, (with levels and lives and Score instead of targets hit), but my flash crashed and I hadn't saved a thing, - stupid me -, I'll fix that cross hair thing right now.
"the shooting of the targets needs to be more responsive, other than that, a good simple game."
Mind explaining differently? I don't quite get what you mean by more responsive.