How can I keep improving after my first 30km ride?
You've got off to a great start, well done. Several answers tell you how to improve quickly, and they're not wrong. But you'll see plenty of improvement just by riding often, occasionally pushing yourself, and resting when you feel the need after a hard effort.
Riding daily to work/school/college if reasonable, with something for fun some weekends is hugely beneficial. It's how I got started, and now I do rides of hundreds of km, having never used a training program. Especially if it's winter where you are, don't feel guilty about not riding if the weather is bad. In the long run enjoying the ride is a good thing, while making yourself suffer is likely to put you off - the time for that may come in the future if you start thinking about specific goals of speed, distance, etc.