Way to go, Bear! Stick with it - the exercise part gets easier pretty quickly. Whenever I start back up again, when I finish a workout I want to puke. Your body just has no idea what is going on! But after a couple of them, it figures it out.
As for calories burned in walking - there are lots of good online calculators - I punched your info into this one
http://www.prohealth.com/weightloss/...culator1_2.cfm
and depending on exactly how you fall you should have burned off between 250 -450 calories in the 20 minutes you did. So that's pretty cool!
And remember, even if you aren't burning a load of calories, there is a huge health benefit to routine daily aerobic exercise. Stronger heart, stronger lungs, more energy, stablized blood sugar. And for a lot of folks around here, when they feel good from exercising it is easier to meet their eating goals as well. Can't guarantee that will be the case for you, but.
Also, don't forget, if you work your way up to burning a lot of calories through structured exercise each day, make sure you are getting enough calories and protein so that you aren't running out of steam in your workouts - and remember, your body will crave more calories if you are working out hard - just remember to feed it the "good" stuff when you are adding back in some calories (lean protein, complex carbs, lots of fiber).
Sorry for the "book" - I really am excited that your journey has begun!