Major DUH moment...I am apparently NOT as slow on the indoor bike trainer as I had thought. I noticed that even though the current speed on my Garmin (Forerunner 305) usually shows in the 14-16mph range at cruising speed, my average never is more than 13-13.5mph.
I also notice that that average speed will frequently drop to the 2-5mph range for several seconds, quite frequently, even though the cadence remains consistently in the 90-100rpm range on the middle ring.
I did some Google sleuthing and discovered that there is a way to shut-off the GPS sensor for indoor workouts. Apparently my Garmin was grabbing a satellite connection here-and-there and it was cancelling-out the cadence/speed/distance sensor from the rear wheel and messing with my average distance and speed--not in my favor. Ahhh...
The way to prevent this is to turn on the Garmin, hold down the mode button until it shows choices of workout (run, bike, other) and select indoor. Now the trick is to remember to set this back to outdoor and running when I do my next outdoor running workout.
Hope this helps someone else. I wish Garmin units shipped with an actual paper manual to reference in a pinch (I'm finding this criticism applies to a LOT of things, nowadays--I'm old skool).
So today's 20 mile trainer ride was likely closer to 21-22 miles. Huh. I look forward to accurate data in future rides. This will be important as I start training for my first duathlon in a week. At least the first month of my training will be indoors...and probably closer to 6 weeks. I can expect half of my 11 weeks of training to be on the indoor trainer. Thank goodness for DVDs and a massive flatscreen TV. That reminds me...I need to go pick up Zombieland!