Wattage becomes unmanageable after a workout or attempting a Just Ride.

During a selected Workout the wattage seems to be accurate in terms of my FTP and cadence but when finished if I try to continue riding, even on a downhill section, it increases rapidly way beyond my FTP and my cadence unwillingly falls away until I basically am forced to stop. Same happens if I just click on Ride Now (Surprise Me or a selected course), I struggle to get going and have to stop too early out of sheer exhaustion from trying to keep moving. I am new to Zwift - am I missing a setting or something?  

Someone who is more knowledgeable than I am will be along shortly, I am sure (I am new to Zwift myself), but I suspect your smart trainer might be in ERG mode which forces you to constantly maintain a certain wattage and varies resistance to ensure that.

When I am riding around the island on a flat at 200w, the game maintains about 34kph which is about spot on for real life riding without wind for me.

Thanks David, I use ERG mode for the workouts. Will try turning it off for a “just ride” next time.