Good question. The short answer, as with many of these questions, is “it depends”. But briefly, between your two suggested options I would take the first one which describes a more gradual progression of volume. Especially if you’re just getting started in running (as you acknowledge), it’s better to creep up on bigger volumes than to assume you’ll absorb ever-increasing 10% gains without issue.
I would also suggest that given your available time before the 50km, you could afford to periodize your macrocycle, incorporate some base speed work right now (hill sprints, strides during aerobic runs, etc) and begin a regimen of muscular endurance/work capacity strength training. Both these modalities will serve you really well as you develop your running and will support bigger mileage in the future. Further, they’ll merge better into a plan which gradually builds volume versus one which tries to increase the mileage more rapidly, again risking injury or excess fatigue.