Although Daylight Savings time is almost here and the days are getting longer, an eight hour day plus four hours of riding is likely to put you in some darkness, particularly if anything delays you. I agree that if you feel a blinky is necessary, a front headlight is also required. Get a good, LED model. Since you will be riding in the dark for only a few hours per week, one of the ones with AA batteries should be fine.
First get your lights, then just do it. You won't really know what your requirements are until you have made at least one trip. I assume that 20 mile bike rides are no novelty for you, so why wait? Do it tomorrow.
Paul