Jafix is right, listen to your body. I'm a certified spinning instructor and did that for 6 years. Their mantra is really "less is more." Most of the benefits of exercise (cardiovascular, immune system, brain chemistry, sleep, mood, healing, etc...) come at the aerobic and not anaerobic level. When you're aerobic, your heart rate is between about 65% and 80% of your max heart rate. Your max heart rate is typically 220 minus your age for men; 225 minus age for women. I'm 48 so that's 172. 80% of that would be 138 bpm as a ceiling for aerobic exercise. Walking is probably the best aerobic exercise there is but your heart doesn't really know the difference between that or an elliptical machine or stationary bike, etc... You build up your anaerobic threshold over time. Most high performance athletes will train at an aerobic level for as long as 6 months until they start training in the anaerobic levels. You also want to keep that aerobic fitness regular; at least once every 48 hours or you'll rapidly lose it.
After 20 min of aerobic exercise, 50% of your calories burned will be coming directly from your fat stores. One trick is to do a brief exercise like walking in the morning before you've had your breakfast. Your glycogen stores will have been more depleted during the night and you'll start burning fat quicker.
So if your goal is weight loss, the aerobic exercise is really good for that. Of course the anaerobic or strength training will put on muscle mass and that raises your basal metabolic rate (muscle burns more than fat) which also helps with fat loss. But a lot of people go nuts on a spinning bike or they go running (anaerobic) or something like that (thinking harder is better) and that can be counter-productive. You finish your workout feeling like crap and then have huge levels of food craving, stuff your face and end up gaining weight.
That's why I'm not that enthusiastic about the Crossfit type workouts. Alan is a huge Crossfit fan and it's obviously done him a lot of good but it's not for me. I think they are too severe for most and result in a lot of injury because people don't get the proper form down immediately. Some also encourage competition and that results in over-use type injuries as well as rhabdomyolysis, where the muscle fibers break down and float into the bloodstream and cause kidney failure/death. I knew a girl at my gym a few years ago getting ready for her wedding. She was already super hot and in great shape but in addition to her regular training sessions, would do a Crossfit class. She got rhabdomyolysis and spent three days in the hospital. If she hadn't been hospitalized as soon as she was she could have died or been on dialysis the rest of her life. Of course most who do Crossfit don't get injured but you get the point.
My trainer is not a carb nazi like me but says you should try to get your carbs in before 1:00 pm each day and they should be the good complex carbs like oatmeal, quinoa, sweet potatoes, etc... The whole paleo logic is that for most of our history (99%) we were not carb eaters for the most part save for the rare fruit, honey or root. Agriculture changed that but in a world with scarce carbs our bodies are adapted to immediately converting that into fat for storage for the winter or whatever hardship. So instead of 10% of calories coming from carbs, many get 60% or more from carbs hence why we're so fat. Chemically, you cannot even store fat unless you have a carb available. There are many poor cultures around the world where people are still fat even without fast food available and that's because all they eat are carbs for the most part. Sure, Asians eat a lot of rice and are generally thin but that's also because they're starving; their average caloric intake is much less than an American's.
The disease theory of paleo goes that every plant has evolved a toxin to keep it from being eaten. Some will kill you, some will sicken you, some will sicken over time. Also some are more sensitive than others but it's these naturally occurring plant toxins that generate an inflammatory response where the body produces cholesterol as a kind of internal band-aid to address the inflammation. Eating cholesterol doesn't produce cholesterol in the blood; it's an inflammatory response. The inuit natives eat thousands of milligrams of cholesterol every day in marine mammal fat (whale and seal blubber) but they have some of the best blood of any people in the world. The notion that eating bacon or red meat will give you a heart attack is nonsense; you're more likely to get a heart attack from eating bread, pizza, corn chips, soda pop, etc... All fats aren't equal but that's a topic for another time.