I can't remember if it's been displayed for practice session specifically. That's one of the trophies that you're going to get automatically - get around the track 3-4 times each race and you'll have it by the end of your 3rd career mode season.
You can simply grind practice miles in GP mode, I guess 80% of my practice miles were achieved through GP mode - got the tropy perfectly fine during the first race or so of the second season.
I'd complete career mode first and see what's left - I got it in career mode but there's actually no way I had covered 805km by that time in career mode only.
And just so you know, make sure to finish the session. If you do 150 miles in P1, then go to P2 and do 150 miles, you'll have 300 miles. However, if you do 150 miles in P1, then hit the restart session miles, you'll have 0 miles completed.
jcmYes you can skip to next session, and you can probably retire to paddock. That said, I got this trophy prior to the 1.02 patch, so their could be glitches involved. You can check your total distance in the stats screen, and if it goes up after the practice session then you know it works.
My total distance is around 15000, though I've provably done about 20000, I just restart the practice session a lot.