Allow for distance customizations based on exercise rather than overall "preferred unit" selections that apply to all.
I think its common to track running in terms of miles. But if I prescribe rowing, its more of a standard to track that in terms of metres. I want to be able to choose the specific measurement standard on current and also custom exercises without it being an overall selection of my "preferred unit" (metric/imperial). Just let me choose the exact type of measurement for each exercise.