Real Racing 3 Endurance: Is there a strategy?

I haven't had any trouble getting 1st place on the Endurance challenges in Real Racing 3 up until now. I'm playing the "Maplethorpe Tyres Precision Driving" Endurance event, and for some reason I can't seem to get more than about 6.5 miles or so before the time runs out, no matter how well I drive the course. And yet I'm seeing there are people out there logging 10 miles or more. My cars aren't fully upgraded; but could that really be the 3 to 4 mile difference? Up until now I haven't had to fully upgrade my vehicles to stay competitive so early in a series (I'm only 34% through this one). I even skipped ahead to the next couple of events and won them easily. So what am I doing so wrong on this one?