How much walking will make a difference?

I'm overweight! I am 4'11 (basically 5ft) and i weigh close to 195lbs.
i was on certian medications throughout the years – that i recently stopped taking- which caused me to gain A LOT of weight. Since i stopped taking the meds, i've lost 20lbs. I'm stuck now.

I think the main problem is that I do not get enough exercies. I work at a childcare center, so im constantly moving/running with the kids … and i park on the 7th floor of the parking garage at school JUST so that i can walk up the stairs when i go back to my car.

I need more exercies, but its hard to find time. I want to start walking. I do not want to do weights quite yet because I do have A LOT of muscle. I'd like to loose some fat before doing anything else.
I have 3 siblings that are basically my life, my responsibility. If i do go walking, they have to come…So we wouldnt be going all that fast. (It will probably be the kids who decide how fast)

How many hours a week do you think we should spend walking?
I should add….. i say how many hours a week, and not a day, becuase there are some days where i really can not walk. For example, most days during the week, i work from 6AM and as soon as i get out, i go to school until 9pm.

Some days however, i have a lot of time (saturday and sunday i have all day….monday and wednesday i have 1/2 the day)
So rather than aiming for "30 minutes a day" or "1 hour a day" I want to aim for "3 hours a week" or so.

I'm not sure how much walking will show a sugnificant difference though.

any tips/suggestions = fantastic!
my diet is very good. I have always been a healthy eater… lots of fruits and veggies…always. Thats what i live off of. – Thats why I think its the exercise that im lacking. My eating cant possibly get any healthier.

But thank you for the suggestions