18 - 19 MPH average is not bad at all... how was the ride though, hilly or flat. For some rides like my ride last night 19 MPH is not bad at all... of course it was constent climbing and lots of stops with sprinting.
This is an overgeneralization, but if you can average 20 mph on your own you should do fine in Cat 5. Average speed isn't all, though. You're going to have to do your intervals so you can chase and attack.
20 MPH can do a CAT 5... I hope that is true, or is that only for road races? I can average 22 - 24 MPH on a training ride easily and I do not know if I am ready for cat 5...
In the last month I have increased my average on our usual night ride quite a bit (2 MPH gain in 1 month). The reason is when I go out I ride hard when I need to and build arobic base when I do not. For me 18 - 19 MPH is base building speed. Honestly the best way to build speed is to ride with people who are faster than you are... that is what I did and look where I am... I can hang