> 10% of that market -- 8 million dogs in need of walking every day
Although 10% may, ultimately, be a good estimate, it's a bit misleading to equate all owned dogs to the whole "market".
At the very least, you'd want to exclude rural dogs, who, presumably, have plenty of outdoor access. Probably pro-rating this for some definitions of "suburban" would make sense, too.
I'm not sure how multi-dog households would work in terms of pricing for a dog-walking service, if an owner wanted them all walked at once, but it seems better to count dog-owning households rather than dogs.
Most importantly (and this could obviate the need for any of the other filtering), the real market is only those owners who can afford to pay $20-$30 for a walk. Of those 80 million dogs, how many are owned by people who can afford an extra $2k-$11k/yr on top of basic pet care expenses (<$1k/yr)?