Strap in if you want to get this achievement because your going to spend a lot of time doing this. First thing I did was start the western reign scenario and coupled the carriages in the station then saved my game to cut a bit of the time so I didn’t have to drive out the station every time I played the scenario. Then I drove all the way to Bishops Lydeard and quit to the main menu. From there click on last played and you should be back at Minehead ready to go. Simply repeat this process and eventually you will get this achievement. I found every time I did the scenario my progress went up by 2-3% so it’s going to take a while. I recommend you watch a movie or catch up on Netflix while doing this as it can get quite boring.
UPDATE After the recent update pressing "Last Played" does not count miles driven. Thanks Ross NCC1701 for the information.