Is there any reliable information on what levels of physical fitness and proficiency were excpected from an infantry soldier in early modern (say post-Napoleonic up to WW I) armies? Were there significant differences? Did the same standard apply to reserve formations as well?

http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/APRT_WhitfieldEast.pdf

So the US Army conducted a study on literally this exact thing. In the context of trying to determine how well the various forms of the PRT had worked.

In general they found that during peace time the need to be fit to deal with the rigors of battle took a back seat to other training and functions. And then when war came reduced efficiency was the result, but fitness would again be less of a top priority after the war and the cycle would start again. In addition having to get unfit recruits in large expansions of the military always taxed resources and slowed growth.