First off let me start by saying this isn't a guide a such but more some advice based on my unfortunate experience of this trophy not unlocking.
The requirements for this trophy are simple enough you would think but it would seem that if you haven't got this trophy by the time you have unlocked this
Then you may be locked out of getting it, by the time I had finished 3 seasons plus the online I thought I'd be not too far away from this given there is no milage tracker
I started by doing flat track races in VR46 WORLD on the first track "MOTORANCH" with my custom rider an length set to 20 laps, at 1.5 miles per lap 30 total per race, each race takes between 35 and 40 minutes, initially I had marked off that I'd completed 10 of these before realising there was a problem but continued on for another 5 races before I decided to try it on an alternate account with a fresh save data, as such I was able to track the mileage under the rider bonus "PHYSICAL CONDITION" which is for racing 500 kilometres in any discipline.
From the start you are forced into the first career race which is a flat track race on the above mentioned track which I did at 3 laps which showed in the bonus as 6km, from there I went to VR46 WORLD and completed 5 races at 20 laps which increased the bonus upto 233km, the next race I just did a 10 lap race giving me the trophy in race once I'd reached 250km, the final bonus stat showed as 256km.
Sorry if this seems long winded but hopefully it may help even one person avoid this issue I had and if there is anything I can add feel free to comment thank you.