Units selection should be available at the activity instance level. User settings would be able to apply as the global default for units of new instances. User would then be able to choose SI or Imperial units within an activity entry that would be saved as property of that instance.
Use case is that the running sport encompasses performance benchmarks and training intervals covering both Imperial and SI units. Training Monday could be based on mile sprints, while training Wednesday could be based on 200M/400M/800M cutdowns. The requirement to apply units at only the global level is not practical for these common methods of training.