I have lost quite a bit of weight, and my thighs and upper arms now look flabby. I would like to try and add some size/muscle to the areas to make them appear more toned and less flabby.
What are the best execises to build up/bulk up the thighs and arms?
I'm currently doing turbo jam, which is a cardio/kickboxing workout.
I agree! Doing cardio is great for losing fat, but to get your arms looking good, you will need to lift some light weights or do specific arm exercises, like tricep dips. Try to do exercises with lighter dumbbells that will work all of the arm muscles. Some people JUST do bicep curls, but you definitely should work the sides and backs of the arm too.
I find that cardio usually is enough for my legs (I like to go jogging or use the elliptical), but I also do lunges and leg presses for my legs. You can also stand up and down on your toes to work the calves.
I hold dumbells and do walking lunges. They are awesome!
Yes, pushup are great for arm toning!!! I am horrible at them, but they work great!!
Again, congrats!! I admire your hard work!
Building up legs and arms... 
