Training runs can be 20 mph ave over 60 miles (flat) - usually 18 mph on hilly stuff..at weekends..... with traffic and lights and stuff but on a road bike...
Commute, urban with traffic, is usually 17mph average on a slick MTB.... I do ride at between 19 and 23 mph on the MTB in between.....
....but I used to race from the age of 18 until 30, and still can't switch off....now 39.. only because I've been back to commuting every day for the last 18 months has my speed got back to the above rate.... took about 6 months to get back to speed.
If you want to get fit, ride as hard and fast as you can, rest at traffic lights, start again....it's a killer, but you get used to it.........not. (But bear in mind traffic, so ease off if busy......)
PS Goffins...it's the engine that counts, a £500 road bike isn't massively slower than a £5000 road bike...the diff with a Focus and an Enzo is the engine....... and some flash bits. I'd say less than 1mph...... I own an Ultegra level bike and a dura ace bike..... both are very similar in speed, just the better bike rides more fluidly and climbs better.....