Just thought I'd give you a few tips for this game. I'm chipping away at it, it's one hell of a grind! My hired driver is helping me out as I write this!
Season 6 in FASTEST possible time!
Firstly, Level 50 seems to be easy to gain within Season mode, most people I've seen have gained the achievement at Season 4 or 5. My achievement popped at the very end of Season 5. I reckon it could be gained in Season 6.
Right, the trick is when the computer gives you the choice of three events to fill in your calendar between Championship races, you press the Y button to choose a new car and it will offer up new choices based on the car you are currently in! Then you simply choose an event which has the least races and the least miles etc and off you go!
If you don't like the choices then just hit Y again and choose a different car class and see what's on offer. Note that it doesn't always change all three events, but there is always at least one better option!
You won't gain as much experience this way as you would doing the longer races but as I have said Level 50 is easily yours long before the end of Season mode so no need to worry. I would also say it is a good idea to use quick upgrade where available as pushing the car to the top of the class helps to complete races more quickly too!
The point of all this is to have all Seasons and Levels under your belt leaving you with all the longest races which can be accessed via the Event List where you can choose whatever you need/want to get finished and send your hired driver on his merry way to your Solid Gold achievement while you enjoy other things!
For information I'm almost done with Season 6 and have used the hired driver to knock out most of the endurance races, longest of which took 1hr 10mins approx- I was out shopping and walked in to watch him cross the line!
I'm about 32% complete if that gives you an idea of the scale of this game!
If you need any help then just ask!