It has a governor.. The P pump 12 valves (and many other multi-application diesels) come with either one of two different governors, an automotive one which has a high idle and low idle, but unrestricted fueling in between. This is what you want in a car or truck where you're controlling road speed with your foot. There's also the "industrial" governor that essentially maps lever input linearly to engine RPM, and endeavors to maintain its set RPM independent of load. This is the kind you find in tractors, generators, boats, etc.

These governors are basically mechanical analog computers which use the inertia of flyweights, springs, and some very clever linkages to do their thing.