Powertap will calculate cadence I already know.

But will it do speed? I want to use a trainer and know how far I have gone. WOndering if I can get away with not buying a speed sensor (I use GPS Garmin on the road outside).

Yes, at least the G3 will do speed. The cadence reading isn't great according to Saris so you may still want to run a separate sensor.

The cadence from the powermeter will have some very high outliers, in my experience. Like values over 250. Kind of throws off your graphs if you have values like those.

Most people I know who train indoors just use time. Just wondering why you would want to track simulated distance.

With the resistance curve of the trainer, you can track power.

But if you're already using a power tap, I dunno.

I just want an idea of total miles for the season.

On a trainer, the concept of "miles" becomes extremely vague. You can crank up the resistance, downshift and end up at the same power/effort and half the speed (or vice versa - set resistance to low and spin at 25 mph without breaking a sweat, accumulating massive though meaningless mileage).

my trainer doesn't have adjustable resistance, and actually is slow compared to flat road. So it means something to me.

Easy. Take miles on the bike and add zero, as miles on trainer equal exactly zero. Miles is a pretty irrelevant metric. What counts is duration and intensity.

If miles it was does it for you, you can simply multiply trainer time by the speed you average while riding. That will give you a number as good as anything.

My trainer bounces around on the floor when I'm sprinting, so the miles traveled isn't exactly zero. Unfortunately, I can't pick up GPS signals indoors.

All the OP wants is a simple answer to a simple question.
YES it will do distance on a trainer. Both the yellow head and Joule (both versions). I would guess that any ANT+ head nowadays does distance.

It would be more useful to track work (kj) or even more useful to use a program like Training Peaks and track training stress score.

In my opinion, of course.

Since I am using a POWERTAP, I have power data. And I am analyzing it on Golden Cheetah and also tracking with strava.

Is it a crime to track your miles as well? Trust me, 1 mile on MY trainer is more work than the equivalent speed on the road.