For most guys, Its not my first morthod of cardio for fat loss as it is more aggressive and more likely to tap into muscle. I try to use the least amount of cardio to make the biggest impat. If a guy is overly fat, then HIIT might be something I use sooner. HIIT cardio does work and works well. You may find that it works well for you. At 15%, you have some room to lose some fat, so give it a try and I bet you see some results quickly, but watch your body and if you start to get too stringy, maybe back it off a little
For building better upper legs, I would first rely on your resistance training and if that is a goal, I would focus on making sure that you are using the intended muscles during your weight training. Often times, guys that need better legs aren't activating muscle in an optional way during their lifting