Just Drive!
You drove 100 miles (161 kilometres) without unlocking any other trophy.

This could unlock naturally during the DiRT Tour as it did for me. Or, it could be the last one left apart from hitting level 30 online, during which it will almost certainly pop unless you hit level 30 before this trophy pops, in which case you must race 100 miles again as it will reset. My advice, dont think about this trophy, there is a good chance it will come by accident. Online miles do count towards the trophy.