A conventional wisdom is that to preserve muscle, you need to exercise
those muscles. It also states that endurance exercise does not help
retain muscle that is not being exercised by that exercise.

http://www.weightlossforall.com/aerobic%20exercise.htm

You could also exercise on an elliptical machine or ride a bicycle,
which is easier on the knees.

Cardio is useful even if it does not help to preserve your muscle
mass, it is good for blood pressure, insulin sensitivity, and all
kinds of other things. So it is worth doing, and resistance training
is also worth doing.

i

At your weight (and mine!!), walking is just as beneficial and you won't
hurt as much the first time.  Start with a 10 minute walk every day and
build up to half an hour.  Then either increase your time to an hour a day,
or pick up the pace until you can't both walk and carry on a conversation
for lack of breath.

Another thing I learned years ago at college from an expert, was that when
you jog, jog loose.  Don't clench your fists and tighten up your arms and
bang your feet on the ground.  Let your hands dangle, let your arms flop
loose, step gently but fast.  You'll have less chance of injury.

You'd be better off lifting weights and making sure that you have lots
of protein in your diet.  Running is OK but picking up heavy things is
what will really help keep muscle as you lose weight.  See
www.stumptuous.com/weights.html for help.  Good luck!

Neil

Any exercise (aerobic or anaerobic) will help avoid lean tissue loss. However,
the primary advantage of aerobic exercise is that it burns calories.

This amount of running will make very little difference, but it will
improve your aerobic fitness. Improving your aerobic fitness is important,
because this will help you get better at burning calories.

You're making this way too complicated, and missing the obvious, which is
that doing strenuous running will cause you injury now anyway.

However, exercising hard on cardio equipment such as the elliptical or the
stationary bike will help.

For preserving muscle tissue, lifting weights is better than any cardio. (You
may even gain some)

Depends on your level of fitness. One approach is to use a heart rate monitor.
Another is the "talk test" -- you should be able to carry out a conversation
when you're running. Not just short sentences between gasps, but a normal
conversation (or sing your favourite song or something)

You're making this way more complicated than it is. This is a red herring.

Cheers,

As Donovan has said, don't make this too complicated.  You are living and
breathing the Krebs cycle each second of your life.  Don't get bogged down
in the biochemical basis of energy production.  Any type of exercise you do
will increase your calorie burn - the more you do and the more intense, the
more calories you burn and the more weight you lose as long as your dietary
intake doesn't increase as well.  BUT you have to be careful how much you do
to avoid injuring yourself.  Walking is great exercise, and will be gentler
on your inactive body than running.  Figure that (very roughly) you burn 100
calories per mile you move walking or running (others - don't get picky
about the exact figures which have been debated extensively elsewhere).
Walk 2 1/2 miles per day each day of the week and you will burn roughly 1/2
pound per week.  As you build up aerobic capacity and strengthen your
muscles and bones, you may slowly want to add in running.  At that point,
there are a number of beginning runner's programs you can find on the
internet.

Margaret

Doesn't matter what you're burning, if you're breathing oxygen you're
using Krebs cycle. The only time you're not is if the exercise you're
doing is so intense it causes lactic acid buildup, forcing you to stop.
If you want to specifically burn fat do low intensity exercise for as
long as possible and avoid high carb foods.

I've no idea what these enzymes are that are supposed to be released.
Fat burning goes on all the time, so all the enzymes required are active
all the time. The exception to this is hormone-sensitive lipase, which
releases fat into the blood from fat stores. This is deactivated by
insulin released in response to high carb foods.

The 20 minutes thing is how long it takes for the muscles to switch from
using mostly glycogen (quick energy) to using mostly fat (slower
energy).

You may end up doing your joints more harm than good too. Long walks
are the easiest way to lose fat through exercise, although long cycle
rides and swims will work too.

MattLB

It is actually far simpler.  The standard prescription for fat loss is to
create a 500 kcal deficit per day (at least on average).  The best way seems
to be a mild increase in activity and a mild decrease in food consumption.
Creating a bigger deficit will speed the rate of fat loss.  It is not
uncommon for some lean mass to be lost as well.

Running or walking about 2.5 miles per day will get you the 250 kcals in
activity.  Cutting down on soft drinks will get the other 250 kcals.
Remember if you start to exercise, you might need to increase your energy
intake from the low deficit you have currently or you may not feel the
energy to exercise.

For beginning exercisers, I say start slow and conservative.  If you have
not been running, start with walking.  If you have been walking, add a few
minutes of running interspersed with a few minutes of walking.  Over time,
increase the running and decrease the walking until you are running
continuously.  Add time conservatively.  No need to aim for more than 30
minutes a day  4 or 5 times per week to start with.  To me it is better to
walk 2 miles than to run 1 mile at this point in your program.

Running hard early on is likely to lead to injury and soreness and
disappointment.

It took you a while to put the weight on, it will take some time for it to
come off.