Why do my legs hurt after doing a core/ab workout? My sides and abs hurt only a bit.. but the tops and sides of my thighs are so sore.
What exactly did you do? Was it a circuit workout?
Pick a workout with a lot of sit-ups and leg lifts... There are many workouts here that feature exercises, like mountain climbers or all the plank-based stuff, which do not target the Abs unless you focus on them.
I found that sit-ups worked great for me. It's impossible to do them without a lot of effort from the Abs. You can have a look at some of the workouts that I created here if you want.
Try hanging knee raises instead of lying leg raises they isolate the abs better. Also planks.. But all in all having strong psoas is important too which is the muscle where you feel the burn according to me. I feel the burn at the top of my legs too.