The distance between Beijing and Guangzhou is about 2000km
The best way to get to Guangzhou from Beijing is flying. There is a direct high speed train, but even the quickest high speed train takes 8 hours (morning to evening)
Flight takes 3 hours and 20 minutes, assuming no delays. But delays happen a lot in China. With delays you won't be spending more than 4-6 hours. In total
But flights are usually about 2X more expensive than the high speed trains. A second class ticket high speed train ticket costs ¥862 ($125), most one-way flight tickets are close to ¥2000 ($290)
In China most of us use Ctrip to buy flight and train tickets