Twenty one years ago I was headed home from the office for lunch and saw an elderly neighbor lying on the ground. He'd fallen and couldn't get up so I helped him into his house, then noticed his lawn was overgrown. So I mowed it for a couple of years, then one 100 degree day thought about the cleaning service van in their drive. "Why am I out here for free, when they're there 3 days a week, in the a/c, being paid?" So I started charging them. Once I did that my phone rang off the hook from people wanting me to mow for them.
As kids came along I scaled back, as they grew older I let the business grow. Right now I'm around 75 accounts and have to turn down new calls. I'm fortunate to depart the office at 3:30 most days. And some days I get antsy and am able to take half the day (or the entire day) off to get a head start.
I have an Exmark ZTR that's awesome. Stihl trimmers, edgers and blowers. For the smaller areas the ZTR won't go I use the Toro Personal Pace. I have a 22" AWD and a 30" TimeMaster Personal Pace mowers. The 22" gets replaced every 2-3 years.
Over the years I've had dozens of people ask me about mowing for a second job. I tell them if they're doing it for the money, don't do it. The last thing on earth I'd want to do when I leave one job is go to another job. I absolutely love mowing, the exercise, being outside, etc. I do other landcare too, but mowing is what I enjoy.
I've made the mistake of trying to help others get started. 100% of the time they've ended up quitting because IT'S HARD WORK. I'll say this, I don't have many bad days at the office, but when I do, ten minutes after leaving work it's not even a blip on the radar. Mowing takes your mind off everything else.