As others have said, start on 42:16 or 48:18 (they are the same). This gears you for 80 rpm at about 16 mph. Cadence is 5x speed. Then as you get fitter you can drop to 48:17 or 48:16, which is my preferred gear for a flat commute and rolling club runs, this is the gear for 19 mph.
If you are still feeling brave, you can then go to 48:15, which I raced on (geared for 21mph). For this gear, I stripped the bike down, replaced the steel forks with carbon and ran lighter track wheels and tubs. It was too much even on the flat for commuting duties though.
And ride it fixed rather than SS, it is so much more efficient.