I have a troy bilt 42" pony riding mower, it is 8 years old, no previous work or trouble.

Lately, after mowing for about 30 minutes, the motor stalls. Turn the key, and it starts right up, and runs fine for a few minutes, then stalls again. I changed the spark plug and put in fresh gas. No problems for the first half hour or so, then the problem starts.

Hi,
Sounds most like a clogged fuel filter, though a bad pump, if it has one, would do it too.
Peace,
Clarke