I am a 53-year old that has been cycling again for 5 years. On a flat course riding solo I started out averaging in the 16.2 mph for 30 miles and worked up to 18.8 and plateaued.
This Winter I did a 16-week training program consisting of 2x per week of 30 min core/flexibility and 60 min of riding a Kurt Kinetic Road Machine. Initial 20-minute time trial was 18.4 mph, end of 8-weeks was 18.6, and end of 16 weeks was 19.3. The class leader is a credentialed coach and he thought a 10% increase in wattage was a reasonable/attainable goal for me which equates to 19.2 mph.
I am realistic and know that with my athletic ability I will never be a racer, my goal was just to get better so I can keep up with my riding group on long rides in hot weather. I was ecstatic with my progress but have nothing to compare it to other than a Computrainer 2 mph guarantee for a 20-week training program which if I remember correctly is 3x per week. Depending on your starting speed, 2 mph could be a huge wattage increase both in real and relative terms.
So how did I do? I attended almost every session and some optional weekend sessions and worked hard to perfom the drills in the training zones specified which was very challenging. Cost was $400 because I took advantage of the early bird discount and committed to the whole 16 weeks.