Just did my 1st Zwift ride. Im using a garmin speed/cadence sensor and a chest HR monitor.
During the ride, the HR and Cadence matched my Garmin 935 reading. The distance was also very close. Off by less than 1 mile (10 mile ride).
But, the speed was weird. At one point it said I was going 40mph and I highly doubt that. The speed on my 935 never matched Zwift.
I noticed that in the corner it showed the % incline/decline and started trying to adjust my resistance according.
I want to be as close to real world as possible, so how do I gauge speed on Zwift? If the speed goes up what do I do?
Also, I am using a Fluid 2 trainer. How do I know I have the right resistance to know if the power output that Zwift displays is close to accurate?
You watch knows nothing of the virtual elevation changes within Zwift so the speed show in Zwift and your watch will hardly ever match.
Speed within Zwift for zPower is determined by your rear wheel speed and the known power curve of the trainer chosen and that is converted to watts, with those estimated watts, your weight, in game bike used, in game drafting and virtual elevation changes your in game speed is determined.
If you want to be as close to real world as possible you will need a power meter or a smart trainer.
Zwift does pretty well with Z-power. It’s not dead on balls accurate. You won’t feel grades, but you can shift into a harder gear to simulate it. Your avatar will slow down on steep grades if you continue at the same wheel speed and zoom down hills at a ridiculous pace.
If you are using a fluid trainer, it does get harder as your wheel spins faster. The impeller inside the trainer pushes against hydraulic fluid.
You will sweat and you will feel like you are getting a work out.
When you are up for it. A smart trainer will simulate the grades in real time.
For now, just ride on. Don’t worry too much about speed or kilometers. It will sort of be close.
I went from a Kurt Kinetic to a Wahoo Kickr. The Kurt taught me cadence and the Kicker bettered my hill climbing. 
Ride on.
Also, a ton of info if you google “how to get started with zwift”.