As I was averaging 16mph over 50 mile solo rides I found the same, wanted to up the game a bit. What I did was by no means scientific but...
Attack inclines (not long hills obviously ) hit them as hard as possible then I'd ride easy at the top for maybe 1/2 mile to get my breath back and recover, then ride as normal.
Generally, dont hit the ride hard as soon as you get out, ride well within yourself for maybe 5 miles, you get it back and then some at the end of the ride.
Alternatively I'd use a computer, set the display to average speed, then aim maintain a speed above my normal average.
Always head out into the wind, if you push too hard and blow out, theres nothing worse than a head wind when you're overcooked.
I also used commuting to absolutely hammer it, although it was only short rides, but use those to push push as hard as possible.
Over maybe 2 years I upped it from 15 to 16, then 18 mph averages over the 50 miles riding solo. It is quite a quite impressive average...but then just once in a while, some fool
comes alongside and makes it all look so easy
Tbf, in my case I found while I loved it, the thought of 50 miles hammering myself became a negative and I eased back eventually.
Hard hard work is really the only answer (and do get down on the drops or get tri bars, you instantly cruise 1 mph faster anyway)