In Mystery and Manners, Flannery O'Connor writes, “The only way, I think, to learn to write short stories is to write them, and then to try to discover what you have done." This seems equally apt for novels. The writer learns by writing. Since you're returning to writing, you might benefit by giving yourself a warm-up period. Read great fiction. Discuss what makes craft strong with like-minded writers. Experiment and practice with low-stakes writing exercises. Seek out feedback on this work. You might write complete short stories. Certainly there's something to be said for thinking about the whole arc of a conflict and revising to more fully reveal your intentions. You might even use these writings to develop the characters for your novel or imagine deeper into their milieu. But don't put the novel off for too long. In an effort to attain some hazy standard of “improvement," you may never get to it.
Every writing experience develops your skills as a writer. First novels are often ones in which the writer comes to important understandings about how to sustain tension and develop character over the long haul. You will certainly learn valuable lessons from one project that can be useful for the next. Still, each fiction has its own demands and idiosyncrasies. And you learn this by getting into the writing.
You can certainly plan thoughtfully, but there's no way to predict how the process will unfold. No amount of practice before hand will guarantee you avoid major revisions. In fact, it's probably a good idea to plan for major revisions. As Anne Lamott writes in Bird By Bird, a book about the writing process, “Very few writers really know what they are doing until they've done it." This is not commentary on writers so much as on that of the writing process. Writing is an act of imagination and discovery. Get your bearings and then embrace this process as you write your novel.