Everybody, thank you for all the advice. I did get the tractor started this morning. I messed around with the gearshift (shifting into and out of different gears), and I messed around with the parking brake (engaging it and disengaging it a few times), and I fiddled with the yellow PTO button that engages and disengages the bush hog. Then, the engine started, no problem.
In retrospect, I think maybe my problem was that the parking brake wasn't fully engaged. I've now developed a better "feel" for how it works.