Anything is possible!
I would say a decent number of my students are looking for that kind of improvement while taking a course with us - and that's about the same timeframe (3 months, including some time for review after the 9-week course is over). A decent number of them do achieve that kind of improvement - though, of course, you'll have to work very hard and even then there are no guarantees in life. Just give it your best.
What is your actual study plan? How are you planning to learn, for example, the grammar rules that you need for SC? (Note: the answer is not: by doing a million questions. That's a really inefficient way to study!)
Next, let's make a distinction between practice materials and study materials. The Foundations books are study materials: they teach you what you need to know. Practice problems are to practice what your study materials teach you to do. So you are going to need to identify other study materials besides the Foundations books - they won't get you to a 680.
So start with some research: Do you want to take a live class? Do some kind of interactive self-study? Work out of books and devise your own study plan? All of these are study materials (and then I would recommend the Official Guides or other official-source questions for your practice).
Do you know anyone else who wants to study now? Can you set up a study group or all agree to take a class together or something? It's great to have other people around to keep you motivated (and you can ask each other questions, too).
If you want to work on your own, then read some things to help you develop your study plan. Start here:
https://www.manhattanprep.com/gmat/blog ... our-score/(that file contains links to other articles; follow the links)
This next one is linked in the above article, but it's so important, I'm linking it again:
http://tinyurl.com/executivereasoningTiming:
http://www.beatthegmat.com/mba/2016/02/ ... n-the-gmatPlanning study sessions:
http://www.beatthegmat.com/mba/2015/07/ ... s-say-whatFeel free to check with us regarding your study plan or anything else that you'd like.