Terrain does effect pace, but any tested ruck march I have personal awareness of adjusted the times to account for the terrain. Further a simulated tactical ruck march differs from a physical fitness test ruck march. The physical fitness test ruck march is seldom accomplished on difficult terrain.
The I have seen ruck routes where every participant being tested had to run doesn't identify who established the route and who established the maximum allowable pass time. I have seen ROTC units implement some stupid and dangerous ruck march requirements. Primarily because the ROTC cadets were not properly conditioned to participate in doing the ruck march.
I also have awareness back during the 1980s and 1990s of TACP, SERE and Security Forces (Security Forces the OPR and operates the Air Force pre-Ranger prep Course for most of the Air Force) doing time tested ruck march silliness (participants were not properly conditioned) just for we are the toughest gratification pat on the back reasons. But this was back during the 1980s and 1990s before operational risk management policies and requirements began to be imposed on such training and testing activities. Also these Air Force career fields are very influenced by see US Army Rangers do this, we must do same with no awareness or appreciation of what they attempted to copy being a simulated tactical ruck march or a physical fitness test ruck march.
I've participated in some significantly demanding ruck marches with US Army Rangers in field exercises, but have no awareness of tested timed course (training route) or fitness test ruck march ignoring the terrain in determining the must complete by time. Aerobic Capacity Influence on Load Carriage Performance and High Intensity Activity Recovery
Load carriage capacity depends on many strength, flexibility, body height and body weight factors, but ability to accomplish long distance forced marches (time limit to a given distance establishes how much energy expenditure duress in the forced) is most influenced by aerobic fitness which is most effectively gained by running.