If you're a full-time student and you're a diligent self-learner, I think it can be done. But beware - it's not always easy and for some it's in fact very difficult.
Greek is a very difficult subject for lots of reason and one of them is the amount of studying time that's needed in order to do well. My experience with studying Greek went like this. In first semester Greek there were 25 students. After a few weeks it was down to about 12. Then 6 students the second semester, 4 students the third and there were only 2 of us by second year second semester. When I was studying classics I studied Greek 4 to 6 hours everyday.
How much time and effort you'll have to put in will depend upon your natural ability to set things to memory and your language/grammar skills. Learners who have studied other languages I think are always at a better advantage.
I think the real challenge will be for you to stay on course everyday. That's why classes work so well, they're great at making you prepare each and everyday. Quizzes, Exams, oral lessons all force a learner to work really, really hard.
You'll need to develop your own study plan to replace all of that.
So yes, it can be done - but I don't think you'll be successful unless you work very hard at it. Harder than any other course you're taking.
As for textbooks - I like Mounce very much. It's an excellent choice and well written for beginners. But find out too what the first semester students are using. Do you know anyone in the class? I would get a copy of the syllabus and try to study with them if that's possible. With the syllabus you can at least get a handle on the pace you'll need to study at.
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