Lambsie-
All good info so far. Let's try to wrap it up.
With any given fuel load onboard the a/c, you can only optimize 2 things - greatest number of hours and minutes aloft, or greatest number of miles flown. The speeds (IAS) for both are very different.
As pilots we have to make the distinction between theory and real life. We also like things to be very simple to understand and use. In theory, one IAS would be optimum. In real life, altitude changes during drift down, so there would be changes to the DD speed. In holding, buffers are built in to account for G loads in turns and turbulence. For my a/c, I've seen significant differences in the holding speed charts from different airlines.
For this reason, your holding speeds would be somewhat higher than min drag. (Hours and minutes.)
If you have access to the Specific Range charts for your airplane (your tech section probably has them), it becomes quite evident that the max range (NM per # of fuel - miles flown) is much faster than the min drag speed. For example, on my a/c (DC-8) at FL 350 ISA, the max range speed is M.74, long range cruise (99% max range) M.78, holding M.67, max endurance M.58.
(These speeds would vary, of course, with altitude.)
The changes in speeds are not linear with altitude changes. From SL to around FL 200, there is little change in IAS for stall speeds, but as you near aerodynamic ceiling, the increase in IAS is more rapid with increases in altitude.
In summation, L/D is much slower than max range, holding speed is faster than theoretical L/D in real life.