If you want to be able to ride 30 miles comfortably, it will start with slowing down a bit and riding 30 miles. Instead of attempting to do it at 18-20 mph, slow down to 16-18 mph. When it starts to feel a bit more comfortable, increase your speed.
And if you want to be able to ride 30 miles really comfortably, start building up your distance. When you can ride 60 miles comfortably, 30 miles will feel like a walk in the park.
So yes ... pick one day each week and work on the long steady distance. You can keep doing your shorter efforts on the other days. And remember that long steady distance does not mean painfully slow. Long steady distance is the fastest speed you can keep up for that distance, and still be comfortable enough to ride again the next day.