I have numerous friends who are making BANK as dog-walkers. These are people who are licensed clinicians, artists, writers, graphic designers, editors, etc.
All of them started dog-walking as a side business to their primary biz, and now do it full time, including boarding.
There are so many people with dogs, that you get to pick-and-choose your pack. Weed out the bad dogs - or, ONLY work with bad/difficult dogs, like one of my friends does - and get paid $20/hr for each of 5 or 6 dogs at at time, 2, 3, or 4 times a day. The money is ridiculous.
Oh, and when your FAVORITE dog, Fido, has his parents leave town for the weekend? You can keep Fido all weekend, and get paid $90-$150/night to play with your friend!
Seriously. This shit is crazy.
Get an older Subaru Outback. Get clothes and treats and leashes. Get dogs. Get familiar with each of the dogs. Bring them together as a socialized pack. Make bank.
Of course, you gotta be OK with weather. People really don't like to walk their dogs as much in the winter/rain/whatever.
There is also a great community of dog-walkers who work together to take care of dog parks, and support each other when one of them wants to go on vacation.
Work as many or as few hours as you want.
Work with only dogs you like.
It's a good way to work with animals, with ZERO training. Just get a biz card.