question about cardio vs weights

Are you increasing your intensity on a regular basis? If not, then you will maintain the muscles you have. To build, you need to challenge your muscles, yes, by weight-bearing exercises. Do you do squats and lunges? That would be a way to start if you don't have access to the gym or weights...

As a general rule of thumb, you DO need to do some weights on legs in addition to what you do on elliptical and/or treadmill, because in some cases, you're not strengthening the legs enough. Doing extra resistance on the elliptical or treadmill isn't going to be enough. Now, lets say you are running a lot at high intensity or high incline, you may not need to do so much. However, you will want to do at least squats, lounges, and if you're at the gym, do a couple weights that will stretch out the hamstrings.

I'm wondering, if you are doing a lot of cardio, is that enough to build up the muscles in your legs, or do you need to be doing weight-bearing leg workouts on top of that? I'm doing upper body and core weight-bearing exercises, but I'm doing a lot of treadmill or elliptical as well. I'm not trying to get bodybuilder muscles, I just want to lose a little more fat and build up my overall strength. Is it necessary to do the additional leg workouts?

Yes, please incorporate strength training.

You're not trying to get bodybuilder muscles? Are you a bodybuilder? If not, I wouldn't worry about it....

Seriously, muscle is extremely hard to put on.......for guys. It's exponentially harder for women.

Are you eating at a deficit, or a surplus. If a deficit, you're not going to gain much muscle (if any), no matter the exercise.

To build up the strength, HIIT, hills, sprints, etc... would work, but IMHO throwing in lower body strength would be very beneficial as far as strength gains go, not to mention it'll make you a stronger runner.