When you install the new title update, the 2015 Mustang is given to you at the start of the game. You no longer have to wait until rank 5 to get this achievement.
Also, as the 50 minutes are cumulative, you do not have to get them all in one go.
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The first Ford you unlock is the Ford Shelby GT500, at level 5. Take it out for a spin. Explore the map, do some events, maybe get the other two Ford related achievements. The achievement will ONLY unlock once you return to a hideout/command post.
Time yourself if you want to.
A little tip, since you can't pause the game, leave it running for 50 minutes and you'll still unlock this achievement.