Lets start with the facts/assumptions
1- Everyone is an individual and no one rule works 100%
2- Age formulas are a conservative starting point for setting HR zones but should not be the end all solution to setting zones
3- We work in two regimes aerobic and anaerobic (physiologically there are others but we limit It to these two)
a. Aerobic (with oxygen) is the normal metabolic state. Essentially we burn fat with a little sugar as fuel
b. Anaerobic (without sufficient oxygen) is a high level energy burn. Fat burn is nearly non-existent. Almost exclusively sugar fuel used.
4- We store about 1000Kcal of sugar as fuel ready for use in the body (<90 minutes at a high level of work). We have 2+ days fat stored in the body ready to burn
. Even a lean athlete with 3% body fat no the roll at the waist is not in a form ready to use.
5- Maximum heart rate (HRmax) is a genetically set
a. HRmax varies little through the years
b. It is a star seeing GI emptying event to find HRmax
6- Anaerobic threshold (AT) is the HR where we change from aerobic to anaerobic work
a. It has a genetic component and maxes out between 85 and 90% HRmax
7- AT also has a fitness marker. It will fall to 50% or lower of HRmax in a deconditioned or ill individual. It is a trainable variable (or is changes as fitness changes)
8- Fat is the fuel of choice for endurance activities but some sugar will almost always be burned during an endurance event. Think of a fat cell as a huge log of energy and a sugar cell as jet fuel. The log will burn long and very hot but a match will not light it off. Conversely, jet fuel flashes instantly with a spark but is gone after the flash. We need a little sugar to keep the fat burring. Oh, and brain food is sugar, also.
9- So at very low end aerobic work (think walk to the fridge from the couch) we burn lots of fat and a little sugar. As we increase the effort (think walk) the amount of fat burn increases but the sugar burn increases just a little faster. As the go into a slow jog the amount of fat burned still increases
What are Zones
So getting that out the way lets talk about zones. We want to start thinking AT as the anchor for setting zones. The reasons are many but the two larger reasons are that it only requires a sub maximal test to find the value and it is the dividing line from which workouts are separated. One way to think of zones is aerobic zones and anaerobic zones. We use five zones; three aerobic zones (1, 2 and3) and two anaerobic (4 and 5).
Anaerobic zones are to be run in sparingly and with purpose. I tell me classes to not work out in these zones more than 10% of the time. Think speed and power. Zone 4 is AT 110% AT. This is the zone upper end short 3-5 minute intervals may be run in. Zone 5 is power and explosive moves and is 110% AT to HR max
We have three aerobic zones a warm-up/active recovery zone, aerobic development and aerobic endurance zones. The ranges for these are Zone 3, 90%AT to AT; Zone 2, 75% - 90%AT; and Zone 1 between 65% and 75%AT.
For the purposes of this we will look at two zones 2 and 3. Zone 2 is where you should focus you long run. Lets say you are running 8 miles try to set a pace where you start in the mid to low zone 2. Adjust effort to keep in this zone for much of the run. Its OK to allow the HR to drift up during the run. Usually around 75-80% of the way through the long run we go ahead and allow the HR drift to move into zone 3 if we feel well.
The weekday work is mostly a zone 3 workout. You want the mid to end of each interval effort to be in Zone 3; the closer you can peak near AT the more benefit you are seeing.
Back to the original question
All zones under AT burn fat. Fat use increases as heart rate moves up through zones 1, 2 and 3. It plateaus somewhere in the last 2-4 beat range near AT then fall to zero in the 1-5 beats above AT. Sugars burn rate starts to increase just after you start to move as HR increases the rate of increase in the sugar burn rate increases
at a faster rate of increase than fat. At some point in zone 1 up through low zone 3 the amount of sugar burned passes and becomes greater than the fat burn rate. In other words you are burning more sugar than fat. That point is in our zone 1 to low zone 2 is where gym equipment will attempt to set your HR for a fat burning workout. And true if you work at a point higher then the 50-50 mix of fat and sugar you are burning more sugar than fat. But that leaves most of zone 2 and all of zone 3 to work in where you are actually burning more fat than the 50-50 mix.
When I get a deconditioned person in we will stay in their zone 2 for a few weeks so they can get their legs. But as soon as we can we get that person up into zone 3 in a hurry. This does a multitude of things. It burns more fuel and fat. It pushes the AT up and it pulls up the 50-50 point where sugar becomes the predominate fuel. If you work with a well designed plan at the intensities pointed at you can push your AT up to near the maximum genetic point possible AND you can adapt so that the point where sugar becomes the predominate fuel to within a handful of beats of AT. Now this is HUGE. Imagine running mostly on fat near your AT for an entire marathon.
All this to say that you should run long runs starting in zone 2 and work your mid week stuff in zone 3. Oh, maybe once a week or so have fun and push a few intervals into Zone 4 to help with pulling you up any late race hills that may pop up. Slipping into zone 4 on intervals can activate a certain type of muscle cell and make it think it is an aerobic type fiber giving you just a little more in the tank.
OK How do I find AT.
There are two ways. One is to get a metabolic assessment at a club offering them. It can be costly in the range of 120-170 for the first test but will give you a complete profile of where your AT is and what your moving Kcal profile looks like.
The other way just requires a treadmill. Hop on and warm up for 3-5 minutes. Set the mill at a pace you may be able to hold for 10 minutes. Forget run/walking for this test. ONE IMPORATNT RULE!!! Do not hold on during this test.
Once you get at that pace note your HR and say a 6-10 word sentence. A couple minutes later say the phrase again. If successful, increase the elevation 2% and run for a couple minutes. Phrase time. If you are still able to talk or you are not feeling like the run is super hard then up 2% and again run 2 minutes. Test again. We keep increasing elevation as before until we find the following. At some point in here you will get to the point where the run is hard. Keep going. We are looking for a couple markers. The first is that you can only say 1-3 words of the sentence per breath. Somewhere in this area you are feeling like the run is very hard and start looking for a way off. KEEP GOING. Once that occurs we then start looking for the point where your jaw starts to open wide. You could not close the jaw if you wanted this is the body attempting to maximize air intake (i.e. changing from aerobic (with O2 to Anaerobic (without)). Attempt to run another minute at this setting and note the HR. This is your AT.
Note the AT and set your ranges.
Good luck. There is a lot here and even more that one cannot really put in a forum post.
So here's my question. What heart rate zone is best to both lose weight/burn fat AND increase my aerobic capacity?? Are these two goals mutually exclusive? I've always believed that the best zone to lose weight is 60-75%. Is that too low to also increase my endurance for running? I'm using this 5K as a stepping stone to running the Princess Half at the end of February, so I definitely need to increase my endurance substantially.
I especially appreciate the chart you posted! 
