I remember running into this challenge back in Gran Turismo 3! This was probably the first encounter of racing knowledge that I needed in order to win. Took me awhile to recall what I did, but in my head it was a compromise between top-speed and acceleration; depending on the tack, its best to do a trial run and survey where in each part of the track should your most top speed can reach, although, most of your acceleration/deceleration work should also be greatly considered in non-straight roads.
Tune your transmission to cater the track and your car's output potential, it's not all about how much engine power you have to output. THEN, as you confidently pass most of your opponents, don't get too excited and slack off because your tire wear will definitely deteriorate in your last three laps, so keeping your TCS on 2 or 3 and later to 5 is best. The Aston Martin will try its best to stay on your tail until the second last lap. Just stay focused on maintaining distance and your lap record then you'll definitely get the gold.
I forgot to mention, but I used to the pre-ordered Mazda RX vision for this particular race.