Once you have reached the LIRR level requirement (for the achievement) then the quickest way to reach 700 miles is to create a custom route down the Achen - Koln ICE route using the M7 and just run it over and over.
(1) I don't think miles travelled whilst speeding count so being as how this route has a 25+ mile section of speeds between 99mph - 155mph it's quite easy to manage the 105mph top speed of the M7.
(2) Miles travelled only register after the journey has finished (after the stats screen) so you can't travel to the 99mph section, save your progress and keep reloading from that point onwards to speed things up - you have to complete the entire journey from A>B.
(3) If you travel within the speed restrictions each 40 mile run takes around 30 mins.
Alternatively you can use the M7 to reach driver level goals at other locations. I used a custom 4 station scenario (with passengers) along Peninsula Corridor and although it took longer to travel the 40 mile route than the the above option I also received 10,000 AP towards my driver level 20 achievement.