MacMillan's book is incredibly weak on military matters so I'm surprised you are bothering with it.
Yes, it was widely recognized that a protracted conflict would be exhausting and debilitating which is why all of the military planning of the period
stressed rapid mobilization, deployment and attack to produce decisive results as quickly as possible e.g. the Franco-Russian military pact
dictated an combined offensive no later than the 3rd week of mobilization.
It's hard to temper public expectations when the public wasn't informed of Britain's military obligations: the Secret Anglo-French General Staff
discussions weren't revealed to the Cabinet until 1912 and Parliament wasn't informed until August 1914.
Of course, German intelligence picked up on the discussions as soon as they happened and Schlieffen's final war plans featured the BEF from 1906 onwards.
And the German public was generally aware of the forces arrayed against it (which would only increase with British military reforms and Russia's Great Program)
and the and the fact that a protracted conflict meant the defeat of Germany and the Central Powers.
