I'm thinking those "2mph x 4-6hrs" comments are about a 24v system, but
I know my 16' ProCraft w/90Merc will last for several hours being pulled at ~1mph by my 42# thrust Motorguide 12v on a Series 29 Everstart battery, and my Hook 7 is running off the same battery (and motor is not tilted up). So, I'm with you on something being off in your case.
You might have the battery load tested. Or if your "spare" battery is the same size/age as the TM battery, and you have the space to do so, you might consider connecting them in parallel. But, I wouldn't do that without testing the Duralast first !! No sense in degrading your spare if the Duralast has a problem.