I fired up the quad to take it for a ride. When I walked by it, my leg got very warm, and I noticed a glow coming from the front of the engine. Upon a quick inspection and an even quicker response to shut it down, I noticed that the exhaust pipe was glowing cherry red next to the cylinder. This quad has always been very reliable, and starts with a quick button push, but this time I had to choke it.

If anyone has an idea of what may have happened, and how to fix it, I would greatly appreciate your input.

 

Sounds like its running lean. That creates alot of extra heat, and results in hotter than normal temps.
Could be old gas, could be the pilot jet is starting to plug up, and is probably why it needed the choke for starting.
Are you using pure gas, or blended with ethenol? If youre using the blended gas, its about 10% leaner than pure gas, and just switching to pure gas may richen it enough.
You could also try adjusting the mixture screw, about 1/4 turn richer, and that will richen just the idle mixture.

 

The plug was a little on the lean side, but I didn't believe enough to cause that kind of heat. It has fresh 92 octane gas .... I clean the tank every year since it's quick and easy to get off. The air/fuel mixture is still at the factory setting....I can try adjusting that. I will also take you on your advice to looking at the pilot jet.
Could the exhaust pipe being 9 years old.... and possibly plugged have anything to do with this? The muffler is a welded unit and cannot be disassembled with cutting it.

 

Quite a few things can cause this.

One; as Hondabuster mentioned, it could be lean. If it has never done this but now it is, it could be a plugged pilot jet.

Two; it could be late timming. I've seen people that burn race gas in a stock engine do this. The high octane gas burns so slowly that it will burn in the pipe and make it very hot. I have not seen pump gas, even premium do this though.

Three; I have seen a plugged pipe do this. It will pack up the exhaust and make the pipe VERY hot.