[Batfish]

From what I'm learning, just about every motor has a "sweet spot" for RPMs and load/gearing, when it comes to brushless.
The more techinically inclined folks should jump in and correct me if I'm wrong here.
The RPM spot looks to be between 35,000 and 40,000 RPM. You should look at the voltage you're going to put through it, then find a kv that suits the voltage to get you in that RPM range.
The gearing I'm used to comes from running on a track, but it's a rule of thumb that can be used for bashing as well. Generally, you set your gearing so that you are only using full throttle at the longest straight, if you use full throttle at all. Full throttle should be reserved for short speed bursts. If you're geared so that you're running full throttle most of the time, you're making your motor spin much more than it needs to to get to the speed you're vehicle is moving. This will cause excessive heat since there isn't enough of a load on the motor. Add a tooth or two to the pinion or get a spur with several fewer teeth.
Oppositely, if your driving area/track never lets you get past half-throttle, you should adjust your gearing to where you can blip full throttle on occasion and drive about 3/4 throttle most of the time. Not being able to let your motor "stretch it's legs" keeps more of a load on it, which can cause excessive heat. Subtract a tooth or two from the pinion or add several to the spur.

I hope that makes some sense.

To sum it up, there are two steps:
1 - Find the motor that suits your voltage.
2 - Find the gearing that suits your driving area.

As far as how this affects your runtime, when you find that "sweet spot" for the RPM and gearing, you will have achieved the maximum runtime for your vehicle in your driving area/track.