I also spread the mileage around. Plus, I am only 40% through my first game, so I haven't got that much mileage to spread.
The highest mileage car I have is my worked-over Evo VI RS. I used it for the second half of the Beginner level, and for the Seattle Enduro. It's got about 550 miles on it. Next I guess is my ZZII, with 400+ miles.
I agree with q_b, though I do also have a lot of cars I've never driven - at least 10 or 12 out of my 35 or so cars. Some that I want to try but never seem to find the time for:
Nismo 400R in platinum grey
R34 V-spec in that weird greenish color
TRD Celica
Toyota Sprinter Trueno GT-Apex Shigeno Version (winner of most-overnamed car award)
Some cars I sold too quickly and wish I had back:
RX-7 Infini III
Mini - in purple!