I am going to presume that you want to lose fat mass. Well, guess what, it does not matter the "substrate" source as long as you are creating an energy deficit.
The ideal body fat for anyone is the lowest body fat that one can achieve and still be able to train(and compete) optimally. 10K runners at the elite level (males) will be 3 to 7% (at least from the data I have seen). The 3 is probably too low and measured improperly.
Running slow certainly gets a higher percentage of energy from fat oxidation, but running at a higher intensity will expend more energy per minute. Since it really does not matter, run however you want or if training mix the intensities properly.
There is a study that shows that the higher intensity may lead to gains in lean mass and a greater fat loss (although the net weight loss was about the same). The study used untrained, obese females so be careful drawing too many conclusions regarding another population, but it does illustrate the physiological point.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=7759741