@ marielle,
I would drive in with your bike, bike home (leaving your car overnight at work if that's ok), and then bike in the next day and drive home. It's less of a time sink but still exercise and less driving. Aim for once or twice a week.
That's how I started biking 19km (~12miles) in the winter, and it was dark both ways... sunrise 8:30am... sunset 4:30pm. I got a super bright front light (non blinky), and a bright blinky back light. I was never worried about assault because I even though it was dark - it was still early with lots of people about. 10pm and rough neighbourhoods though would make me think twice.
Be kind to yourself while your building up a bike habit, start with nice weather days and give yourself time. Just build consistently and try new things, go farther, carry more...