No specifics here, but a lot depends on your level of fittness.
I believe I saw a note that world class women marathoners can reach 50% fat metabolism! I'd be luck to get 10% at that level of effort.
Also, The Arctic 1000 Team specifically trained to increase fat metabolism, and tried to keep their pace appropriate to that. If I recall, Richard Nisley was involved with that.
Bottom line: A lot of science, training, note taking, and discipline.