I started at 40, but with no physical problems except being 25 lbs overweight. My wife has a corporate job, the benefits are a very big plus as you know. You have a huge advantage where you have prior experience. Starting with a ZTR is smart. I learned fast that a WB limits your ability to make money.
Making $40k is possible, especially with plowing. I did $10K in plowing my first year. To get the other $30K, you will need to NET another $1K per week in the summer. So do the math, anything is possible. If you can mow 30 lawns at $50 each, grossing $1500 per week, you can make it. My first year I only managed to get up to 10 accounts, I think. Good luck.