Ive been wondering the same. Ive also been trying to determine if she was really out running. This article,
http://m.telegram.com/article/20160809/NEWS/160809342, and others I have read suggest she went for a walk not a run.
If she was out for a run, why would a rapist take the time to remove her shoes and socks and light her feet on fire. Very odd. Im not saying youre wrong that it was to prevent her from running away, but it would just seem like the last thing someone would do to prevent her from escaping.
I would think tying her feet together would be a quicker option. Also, I dont think most rapists run around with a blow torch or a jar of gasoline to burn their victim.
Plus, if you were going to burn a body with gas, wouldnt you throw the fuel on the person, not just light the feet, head and hands?