Will my thighs get big?!?!

No your thighs will not get bigger. They will be tone and have a nice shape.

as a runner, you get almost all of you energy while running from a process called oxidative glycolosis. Because of this, most of the muscle in your legs is Type I muscle fiber. a non mass building, slow twitch red muscle fiber. If you want to do some sort of training other than running focus on calves hamstrings and glutes with a little bit of quadriceps. Squats, lunges and supine hamstring extensions are good exercises to look at. The key here is to keep your reps at 15 or above and your sets at 2 or less, going for a total of 25-35 reps per exercise. By keeping your reps high weight you're working with either light or just boydweight you will not develop any size and will be using alot of the red muscle fiber you want. also, make sure your exercises use a temp. as in take 2-3 seconds on the negative movement (i.e. the downward movement in a squat), a one second break at the bottom of your rep and 3 seconds on your concentric movement (i.e. the upward movement of a squat). by not performing fast or explosive movement you're removing even more type II muscle fiber involvement and helps prevent muscle mass increases.

WITS/ACSM certified personal trainer

Probably