Just keep playing the Dirt Tour. I got this by my 4th career year about 3 or 4 races in. Also, do some of the specific events that you unluck along the way (Rally, Rallycross, etc) They're worth 5000 xp a piece i believe.
If you have any questions, let me know and I can give more details.
Ginn0rz added: "It's also worth reading the objectives for each race so you can earn the bonus points at the same time. They're often worth up to 1000xp a pop."