I've been trying to lose weight for a while now, and only have about 30 minutes to work out each day. Walking hasn't really affected the scale as much as I'd like, so a friend suggested I start running instead. Do I have to worry about getting huge, muscular, and bulky thighs?
— Looking to Lose
I think it is great that you're kicking it up a notch when it comes to fitness. Your concern is a common one since running mostly works the lower body. To find out if you need to worry about your thighs, just read more.
Running will in no way bulk up your quads into huge masses of muscle. Since running is a great way to burn calories (which is what you need to do if you're trying to lose weight), it'll actually help you lose fat from all over, including from your thighs and bum. So on the contrary, it'll help your thighs look slimmer, not bulkier. Think about photos you've seen of marathoners like this one from the 2009 Boston Marathon. Do any of them have thick thighs?
Running will also tone your booty, quads, hamstrings, calves, and shins, so once you start shedding pounds, it'll reveal sculpted and shapely muscles. The same goes for biking and hiking. These cardio activities will burn calories and tone beautifully without adding bulk. I would suggest you look into ChiRunning. This technique helps you run more efficiently, which helps you avoid overworking your quads.