If you are new to fixed, start with spds. If you are good at clipping in, ride road pedals instead (if you fail to clip in first time, you can’t coast!). Ride with front and rear brake. Slow with the brakes rather than the pedals.
44:18 isn’t a bad gear for the flat. Not particularly high, either
48:18 or 69″ is a standard and geared for 18mph
I’m running 48:16 and it ranges from OK to @#&$! depending on luggage and tiredness. I’m thinking 44:16 is a better compromise for speed and strength.
I’ve never ridden singlespeed on the road (only fixed or gears). Fixed just feels so much smoother. It is fluid, no coasting makes for a lovely bike-connectedness feel. Silence is a plus. But then I’ve never ridden fixed off road (was about to before bike was stolen).