Garmin Bike Sensor... distance??

Yes. 

And as long as your trainer is supported you can train using Virtual Power on most training programs which IMHO is a far better metric to use short of an actual power meter. Check out this thread if you would like to try a free trainer program with the BT cycling plan integrated into it. 

If it's purely a distance/cadence sensor, then it CAN work, if you are fair - but I am dubious of any reliable numbers.

I have explained it this way in the past.  You can crank the resistance way down and push the biggest gear you have on the bike.  The sensor will "think" you are doing 35 mph for the hour saying you have gone 35 miles in your workout at a HR of 140.

OR, you can crank the resistance way up, run a granny gear, and the sensor will "think" you did 6 miles in that same hour at an HR of 140.

So which is right?  Did you go 6 miles or 35 miles?  In real world riding, probably somewhere in between.  Physiologically, your body only knows you rode for 60 minutes at a HR of 140.  It has no idea of how far you went, nor does it matter.

In order to keep my logs reasonable, I realize I had to put SOMETING down.  But I realized that an hour of real world riding at an HR of 148, I generally average 16mph on the roads I typically ride.  So if I average 148 for the hour?  I write down 16 miles.  If I average 138?  Maybe write down 14.  If I average 158, 17-18.  You are probably going to be within 5% of what your real distance would have been and that's good enough for the logs and probably just as accurate as putting some computer on your back wheel to measure some fake distance and it's not aware of the resistance you set it at.