Base training (below aerobic threshold) is always a “more is better” scenario, so it’s really limited by your life constraints.
Rather than a “deload” period, after the base period, I would plan for 6-8 weeks of higher intensity training followed by one or two easy recovery weeks before the trip.
Use TftNA to build the plan. You could expand the 24-week plan in equal proportions so that it fills 36, 40, etc weeks. Or you could do two shorter cycles between now and next spring.