Honestly any run is good if you put time into it. Earning large amounts of money will always be a grind in various ways depending on what runs you decide to do.
Legal runs such as Salt, Sand, Oil and Cement are the low risk - medium earners
Illegal runs such as Cocaine, Heroin, Weed, and LSD are the high risk - high earners
Then you have smaller runs such as Apple and Peach, which help to get you going.
In your case I would hope only having 20k you would own a vehicle with a trunk, possibly a pickup truck - in which case go and gather some apples, process them into Cider at the Cider factory, rinse and repeat until you can afford a larger truck. After that you can do whatever you want really.