Resistance is caused by the jets found on the front wall of the machine. Running without the jets (or resistance) on is kind of pointless as it is far too easy. I recently tweaked my foot and had to lay off from running for a few days so I was forced to do a workout on the hydroworx. So, I set the speed at 8.0 and the resistance at 25% to simulate running 500's at 84 (the workout assigned for the rest of the team). In terms of LT workouts, I would recommend doing tempo type runs on it while trying to find the right speed/resistance setting that would allow you to run at lactate threshold. In my experience, tempo runs are the easiest workouts to do as you are not required to change the setting often which can be quite annoying as the hydroworx does not respond as quickly as a normal treadmill.
I hope this helps.