Many short stories serve as a first chapter of a novel. (Michael Cunningham's "White Angel" comes to mind.)
As a teacher, I allow students to write excerpts from novels. It's no different from grading a short story, flash fiction piece or whatnot. The only thing that you really can't track is plot, but you can mark pacing. Each section/scene/chapter serves as a checkpoint of some kind. I've had students turn in the beginning of novels, middles, ends.
It's all craft.
If they're not getting POV/setting/voice/distance in a short story, then they're not going to get it in a novel either.
Teachers who can't "grade" novel excerpts are either too lazy to try or are too stuck in traditional expectations (i.e. the MFA short story debate) to open up.
Every single instructor I had in grad school encouraged us to turn in novel excerpts instead of short stories. I had one that preferred to get that kind of piece in a workshop. His reason was that novels are easier to sell to a publisher than a collection of short stories.
I did facilitate workshops for other instructors who did not allow for novel excerpts, and the genre students in particular (if they were allowed to write genre) were especially hobbled. Most of the stories were 8 pages of world-building and setting up, then 2 pages of "and then, and then, and then...." (forced climax). If an instructor isn't aware of genre conventions (i.e. world-building) they aren't going to know how to handle that.
I agree that learning to write the short story gave me far more patience as a writer, a love of the scene and the "moment," but that's my own, personal experience. I believe strongly that writers can grow and evolve with the novel process. I mean, the short story hasn't been around forever. Novelists found their way before and they do it now, too.
I will say this, the agents I've researched in the past usually have something to say about getting your work out there before you submit a book. If you show that you're capable of getting a short piece published (print, not just ebook/self-published) then I've heard many of them say it's a sign that they're taking the craft seriously. Not sure I agree with that wholly.
I also went to a book signing where a popular fantasy author likened novel-writing to climbing mount Everest, stating that you don't just "go out and climb Mt. Everest first," you start with the climbing wall at the rec-center.
Sound advice for some, but not all.
In the end, all you can do is look at the suggestions here and find the best one for you. I will say this, it's exciting to see a new writer testing the waters. I'm happy for you and hope you get as dirty/messy as a pig in the mud trying to figure it all out.