Is 25 km/h average speed good? What speed should I aim for?
There is no such thing as an ideal speed you should be aiming for.  It doesn't matter how fast you ride, there will always be faster riders, and there will also be slower riders.  The main thing is that you give it your best and try to enjoy your ride.

At events here in the UK it is typical to see riders averaging from 16-35km/h and everything between.

Edit: Advice above is for a typical sportive ride - if you have intentions to race things are a bit different and you should expect to need to be able to hold a bunch travelling at 35-40km/h for 2hrs for entry level road racing.