Exercise question to those in the know

 You know how at the gym on the eliptical or treamill there is the fat burning zone or the cardivasular workout.

Well I have been hiking, hard mountain 7-8hr hikes which is great, I started finding the gym abit tedious, but was thinking on the way down the mountain today is what I'm doing (hard mountain hikes) best way to burn fat as I'm definetly heaps fit, but my weight is coming down but not as fast as I thought it might of.

I know about muscle mass etc, and have obviously gaining muscle but would really love to keep my exercise in the fat burning zone.

Any exercise junkies out there that may have the answer?

Well I used to be in really good shape, then a bad injury, weight gain... Years... Anyway! I know you've got to break up your workouts, throw your body curves. If you do the same type of workouts all the time you'll maintain muscle memory not real gains. You've got to work different muscle groups, circuit type workouts are very effective. You can actually do that with your hikes. Check out the caveman workouts. They involve climbing, lifting, bursts of cardio (running) very effective and fun workouts.
As you already stated, muscle weighs more than fat, so the scale won't be near as good an indicator of your fitness level.
Good luck, and keep it up! You're doing great!!

you'd probably do well by investing in a Heart rate monitor ( I recommend Polar or Garmin brand) .... and a model that comes with a body strap.... they aid you during your workout to keep you in your target zone

"Target Training Ranges" are not all that useful for a couple of reasons but not the least of which being as you get more fit your heart rate slows.  My resting H/R is 48bps.  You're better off going by percevied exertion.  If you can talk without gasping but not sing, you're working in that 'fat burning' zone.  Having said that,  pushing harder into the cardio zone doesn't mean your'e not burning fat. You're burn more overall calories and probably more total fat in a shorter period of time. It's just he percentage of fat calories used is smaller.  Women tend to burn the most fat and make the best fitness gains by doing intervals of high vs. lower intensity. You probably naturally do that while hiking.  I'm doing intervals on the treadmill jogging at 5.0mph and doing 8.0mph intervals.  When I first started doing intervals like that at 6 weeks post-op, literally I was walking 2.0mph and 'sprinting'  at 3.5mph. 

Building muscle means building metabolically active tissue. Cardio burns calories while you exercise. Muscle burns calories 24/7.  Doing a mixture of strength and cardio ensures heart-health, better bone density, and better metabolism.  

Have you had your body fat % tested?  You may be much leaner than you think or than your scale may indicate.