Your target for level 20 is 66500 xp.
A good way to boost this is with 3 other people
The best way my group and I found to boost was to:
Set up a championship - 12 rounds
Pick New Zeland as all the stages as this is the shortest track and make sure you put it on private.
Whoever is going for the wins will win all 12 rounds and by the end of the 12th stage you will earn will earn around 4900xp including any green sectors you accomplished.
Ideally you don`t want the others racing incase they get a green sector, therefore the other players should just sit on the start line.
You also get a bonus at the end, when one person is going for the wins the others just sit on the start line, one person sits in third place, and the others swap second and fourth place.
Doing this will give 2nd 3rd and 4th the same championship xp.
When you have done a full rotation of championships and when everyone gets there full championship win, each person will end up with around 9500xp per session,
Here is a breakdown of what you get in xp for four places per race
1st-330xp 2nd-270xp 3rd-120xp 4th-70xp.
Hope this helps, good luck.