I'm going to provide a quote of a reply I gave to another user and I'll include some stuff you didn't ask about since it can also apply to your situation even if it isn't an airplane... this user is/was trying to increase flight time, in your case it'd apply to run time.
Think of it like this... You can relate electricity to water to get a handle on it in your head.
Voltage - is pressure for electricity... like pressure is to water.
Amperage - is the current... or amount of flow. The more water that passes a point, is the same as the amount of electricity ( MAH in batteries ) that flow.
Wattage - is the amount of work that can get done by either water or electricity.
Resistance - ie friction ... with electricity, it's the lost energy to heat.
The MAH or AH ( depending on how you want to measure it ) is the amount of stored energy, waters analogy would be the water tank. The faster the electricity flows, the quicker you drain the tank. You can increase the flow by opening the throttle wider, or decrease it by closing it. You can also increase the flow by using a bigger pipe ( large C value ) or decrease it by using a smaller pipe ( smaller C value ). So lowering the throttle has the same end result as lowering the C value, it slows down the flow or current depending on which word you want to pick.
As we increase our throttle on the radio... the resistance values also increase so it becomes less efficient at some point along your throttle curve. So the trick to extending the flight time... is to find where the sweet spot on the throttle is at for your given battery and motor. It seemed on most of my airplanes, the sweet spot seemed to be in the 60%-80% range on my throttle... but it varied depending on which airplane I was flying at the time as well as the flight conditions.
To find your sweet spot requires a few test flights with each one using a different spot on the throttle and flying for the same amount of time each time. Then just see how many MAH you put back into the battery for each flight.