If you have a speed sensor, that's where the distance comes from. In my case I have a Tacx neo, so it's pretty much useless since I could be doing the same workout power-wise at 20 or at 40 km/h. I usually favor low speeds to keep the noise down, so distance will be under-estimated compared to the same workout on the road.

It would be nice if TR could log a "virtual" speed and distance, based on power. Like Zwift does.