Yesterday I had my platinum trophy and I have to say about the 70 races that:
In each category you have to get a certain number of kudos to access the next championship and older, make the builders challenge.
The latter also counts as a race for the 70s.
The road is not difficult, it is monotonous.
If you see that you are not sure of getting the trophy here is a tip:
Make the longer races using the chooper motorcycle since those circuits tend to have many potholes and that bike goes well.
Use the existing ramps and jumps that give you turbo.
This bike is also easier to get extra points for maximum speed that, combined with jumping and rubbing the walls, will give you better points multiplier.
If you see that they reach you, change the bike for the superbike and stick to the wall to get extra points and turbo.
When you make the last builders challenge that gives way to the last championship and you overcome it, they will give you the trophy you want so much if you have made the right way to reach the last championship in 70 races.
Courage and good luck