There are three places you can go from here.
1. Work through other textbooks. Groton's is a good book, but no single primer can get into your head all the stuff you need to know about Greek. There are some books that are more deductive while others are more inductive. You need both. I would say you have to work through about ten of these books. Athenaze may be the best. Do the English to Greek exercises if you have the discipline.
2. Start reading. I agree with Modus that heavily annotated texts are the best. Check out Geoffrey Steadman's Symposium.
3. Use Ancient Greek as a Living Language. Listen to it, write it, speak it. Christophe Rico's Polis will get you started in this. Post on the Agora here at Textkit. Make a You-Tube video where you speak in Ancient Greek.
It matters less where you go than how often and with what commitment you go there.
αναγνωθι, γραφε, ακουε, λεγε. καθ' ημεραν.
I am writing in Ancient Greek not because I know Greek well, but because I hope that it will improve my fluency in reading. I got the idea for this from Adrianus over on the Latin forum here at Textkit.