we got a dog a year ago.
I do most of the walking.
I walk for about 1 hour 15 minutes, and in that time I walk briskly, get warm, red in the face if going up hill and it takes me a while to cool off when I get home.
Over the year, I have speeded up, so it is now a one hour walk, and I have got fitter, so while I always warm up, so I know my heart rate has gone up, I don't get out of breath on the hills, I coudl carry on a conversation all the time.
I still go red in the face though!
I walk about 3 miles.
I would start by finding a route, maybe off road (we have lots fo footpaths and cylce paths through our town, so no need to walk by traffic) Try walking 1 mile, and see. Your goal is increased heart rate, and about 20-30 minutes exercise with raised heart rate.
If that is too far, start shorter, if not enough to get you going (it takes me about 20 minutes brisk walking no to get warmed up) then increase it.
I use runkeeper app on my phone to track what I am doing, you can use it for walking as well as running.