A simplistic, but practical way to calculate total calories consumed is
weight (kg) * distance (km). The amount of time spent only comes into
play depending on the pace at which you can put in the distance. Since
you are walking now, one way to increase your calorie burn in the amount
of time you have is to intensify your workout to cover more distance.
This means going faster. Either walk faster, or (since you posted to
rec.running) start running. Rowing will burn even more calories, so if
you have access to a rowing machine, that's another option. Yet another
option, comparable to running (so long as you keep the intensity up) and
 lower impact activity: elliptical training. Cycling (stationary bike
in your gymn) is yet another option, though since it isn't
weight-bearing, it won't yield too much more calorie burn than vigorous
walking. If you do it, try to keep your RPM at 90 or so for a good burn.
Good luck.

All those calculations are so erroneous that its hard to judge what
your max rate really would be.   You are going by ballpark numbers,
and I'm very sceptical on these elliptical numbers.   For instance, 3
miles on the elliptical at a pace of 6mph burns more calories than 3
miles on the tread mill going the same rate.   And the motion is
equal.  (using an elliptical that doesn't have arm motion).  

Its all calories.   One thing to consider though (and I'm not sure if
you will face this in only a 30 min workout) is whether your reach
that catabolic state where you are burning muscle instead of fat in
the increased workout time.  

A couple other options come to mind.  

1.  Interval training

2.  Weight training

Build yourself a routine where you can do your hard cardio or interval
training 3 days a week and your weight training 3 days a week.    The
interval training you can burn more calories in 1/2 hr than you would
w/ walking, and increase your CV health at the same time.   Add in the
weight training and go for muscle mass gains.  As you know - you will
burn more cals and more efficiently w/ the added muscle mass.

Hello Ig,

 Today I reached just over 1000 calories on an elliptical in 65 minutes.
So I'd say that 500 calories burned in 30 minutes will be easy for you to
achieve.  My personal belief is the higher impact workout is better than the
walk.  Your heart rate is higher and you sweat more getting those nasties
out of your body.  You know I do both, but I don't have the time restraints
you have right now.  When I start back to work I think I'm going to do the
high intense cardio vs. the walk.

Paul
300/203/175

500 kcals is 500kcals regardless of whether it is expended via running, XC
skiing, swimming, etc.

try

http://www.primusweb.com/fitnesspartner/jumpsite/calculat.htm

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 Depends on the machine and the person.  Ellipticals aren't bad
approximations to running, from what I've seen, so equalling your
running rate of burn is not likely to be too hard.  Doing so
by swimming or biking would likely be difficult.  Even though it's
possible to meet or exceed running rates of calorie expenditure
(particularly in biking), it takes technique and experience to do
so.

 Calories is calories.

 Effort level is a much better indicator of equivalences than those
formulae.  The formulae all have approximations and assumptions built
in.  The Borg scale (no relation) is very helpful for assessing your
effort levels.

 If you spend 30 minutes at the same perceived level of effort,
whether it's running, elliptical, ..., then you've probably burned
about the same number of calories.  The actual number of calories
probably doesn't match very closely what the machines claim.  (I've
noticed a distinct high bias on the machines, 20-50% too high.)

 If running 2.8 miles at 7.46 mph represents a race level effort
for you, you'll definitely be toast in short order trying to achieve
that (running or elliptical) 5 days/week.