Increasing runtime means you have to use less power, plain and simple.
Biggest thing you can do (aside from using higher mAh cells) is to use a gentle throttle finger. That's where the majority of runtime losses will occur due to the huge current spikes. Also, increasing punch control can help, but may take away some "responsiveness", which may be noticeable during racing or changing attitude in the air during jumps.
When I'm using my
XT8 truggy (5s 5Ah lipo, MMM, geared for ~35mph) towing my 23lb 18 month old around the neighborhood in her wagon, I get well over an hour of runtime going slow and not stabbing the throttle. Of course, that's probably a little slower than you want to go, but just showing that gentle running can seriously extend runtime.