mibu

have you tried running by interval?
like you run at a high pace for a certain length (such as 300 meters) then walk for a minute for rest, but not too long because you wanna keep your heart rate up.  then repeat

this way you'll achieve a high level of heart rate for aerobic training and the high pace that you want, but you dont have to sacrifice overly exhausting yourself.

if you wanna lose weight in a certain body part, according to science, it is impossible because your body is a 'whole' and cannot divided by chunks and only lose fat locally.  your body will lose fat from all over