That's the best thing you can do, just keep making progress one day at a time.
Although it is equally important to scale projects so that you KNOW you can finish it by yourself.
Trying to make a game with 20+ levels, with dynamic missions, and an excellent music score is a task that'll really strain a one-man team.
For a driving game, I think an excellent idea would be to make a Crazy Taxi/ Simpson's Road Rage clone, that way you can focus on only one level and work from there. But that's just my view on it.