By Aerodynamic Definition, your most efficient cruise speed for time
aloft is the the drag curve minima (point between the induced drag
curve and the profile drag curve), which also conveniently happens to
be defined as Vg. This is the speed at which the aircraft requires
the least amount of energy to stay aloft.

Power back to maintain Vg, lean the hell out of it (At such a low
power setting detonation is a non-issue), and cruise to your hearts
content.

Do not fly "as slow as possible", keep in mind if your airspeed drops
below Vg, your power requirements will actually go back up.

-Scott