Middlemanning the dogwalking trade is as scalable, if not more so, than ecommerce. Gig-economy work is the creation of value through organization and communication.
$600M in VC money is $2/US citizen. If half of households have a dog, that's ~$8/dog-house. If 10% of them want a dog-walker (the biggest "if"), that's ~$80/customer. At $30/walk, $600M in investment doesn't seem entirely crazy.
The median American spends their entire monthly budget on rent, transport, and food.
You need to divide $600M by the 7 million households that own dogs in that area. That's $80 per potential customer. And you need to convert and not churn all 7 million.