It's not a "ton of anything" that will help you lean out the lower body. You need to adopt a balanced approach, not a frenzied one. You also need to realize that there will be many answers to your question and they will all be different because each person replying has found a formula that works for her. So, you will have to do likewise: invest in some trial and error and find the formula of sleep-nutrition-exercise that works for you.
Nancy is correct, of course. Your whole body will lean out and you don't get to decide where the weight comes off.
Diet and nutrition are of paramount importance. As you reach a certain age (around 35), you can no longer rely on youthful ways of eating. Your metabolism changes as you age, and you cannot escape that fact. You can no longer eat any old crap, exercise sporadically and stay up half the night drinking and expect to be lean and healthy. So, if you have any of those behaviours lingering, ditch them right now! Any food that is excess to requirement will get stored on the hips, butt and thighs, so examine your diet now. Is it full of fruits, vegetables, whole grains, legumes, beans and nuts? It should be. Ditch the protein powders, bars and shakes and invest in real food stuffed full of vitamins, minerals, naturally derived proteins, carbs and fats.
Some people live to do cardio, some need to do a ton of cardio each week for the mental benefits they derive. Certainly, as far as cardiovascular health is concerned, you need 3 workouts per week. One can be a HIIT variety. You don't need to do 60 mins cardio per day, unless you desire to or derive such great stress relief from it that you cannot do without it. Find an activity you love doing and you are more likely to stick with it. Find a reason to do this cardio activity that goes beyond fat burning, and you are more likely to stick with it. A sponsored event would help you here. Do you run, swim, bike? Would you like to?
I find that a balanced approach to resistance training helps keep me lean in the lower body. I am tall and lean and find it hard to build muscle in the upper body, but I can develop muscle quickly and easily in the lower body. For this reason, I no longer do Cathe style lower body weighted workouts more than once every 10 days or so. I don't load my barbell heavier than 40 for squats and I prefer the GS legs approach where the reps number varies constantly, there is an emphasis on endurance rather than muscle size building, and the inclusion of low ends and different pacing. I am sure Butts and Guts would be similar, but I have never done it.
I also like to do a different sort of lower body workout that does not use weights at all, but very high reps of low end squats, squats with leg raise done very fast so there is a cardio effect, step ups, running sprints, walking lunges for 5 mins straight, consecutive power kicks, etc. Some people might associate this with a plyometrics type workout and it basically serves the same function. Builds resistance and stamina for the lower body without (much) gain in size.
The third type of leg/lower body workout I do uses the decline leg press/hamstring curl/inner and outer thigh/ glute machines at the gym. The fourth type of lower body routine I do involves moves that blend resistance training (wall squats for 2 mins) with balance work and non-weighted training derived from pilates and physical therapy type moves: cross body leg raises, inner thigh presses with stability ball, stability ball hamstring roll ins, clam shells with resistance band, etc.
I vary all these different sorts of lower body workouts and do 2 of them every 10 days. In addition I run twice per week (6 miles each time) and do treadmill powerwalk intervals of alternating speed (5.2-5.7 mph) and incline (10% - 15%). Both of these cardio activities also work lower body muscles for endurance.
This is an approach that I have found through trial and error and that works for me. I no longer worry about increasing my barbell weight constantly for squats. I no longer do that many weighted squats. I prefer to do walking lunges rather than static lunges with a barbell. I am more interested in lower body resistance training that also contains a cardio effect and cardio that also contains endurance for the muscles of the lower body. Training with very heavy weights may work for some but I prefer this approach.
You need to ignore so much of what is said and written about fitness and concentrate on going back to basics, playing around with different activities to find which you enjoy most and which work for you, and adopting a balanced approach: you don't need endless cardio, a 200 pound barbell and weight training 3 times per week. The body needs rest as much as it needs training. But, you may decide that you like these things! You get to play around and see what works and only stick with what brings you pleasure and that "feel good in my skin" feeling on a day to day basis.
I can tell you that I am never leaner than when I have completed a 2 week backpacking holiday, where the emphasis is on walking a lot, walking uphill a lot, getting way more sleep than I ever do at home and eating a carb rich, low fat diet. There is something to be learned there..........
Hope this helps, as rambling as it may seem!
Clare