Marathon in March-ish?

I ran a 30k (my watch said 19.17mi, but close enough I suppose) yesterday and it was the farthest I've ever run (I ran 15.39mi as a training run two weeks prior). I'd like to work toward a marathon next and then ultras eventually. My issue right now is training time. I attend basic law enforcement training starting in Jan and we have mandatory PT on M, W, F. From what I've been told it'll be a lot of "running" and bodyweight exercises to prepare us for the fitness tests we need to pass in order to graduate. So my question is basically this, if I take advantage of those cardio sessions and push myself during running at PT, do y'all think weekly long runs will be enough to run a marathon in March? The goal is just to complete the marathon, I don't really have a time goal in mind. My 30k was a hilly trail race and I managed a 9:08/mi pace with an overall time of 2h58m.