I would never endorse this kind of training plan for others, but it worked for me back in the day and was (in hindsight) severely lacking in long runs... I ran 20 marathons between fall 1986 and spring 1996 - fall 1986 was senior year of college. I was not a talented college runner, barely sub 30 for 8k and only started XC my junior year (didn't run in high school) although I did race Nordic for 3 years of HS plus all 4 years of college. I ran 2:54 for the marathon, had another under 3 hours, and ran about 6-8 in the 3:00-3:10 range. My long runs in my marathon buildup usually peaked at 15 miles in 1:45 (7:00 pace). I only ran a few 16 milers and maybe did 18 once. Never ran a 20+ mile training run.
What was effective for me when I ran my best times at distances from 15k through marathon was back to back hard days of 90 minutes, done about every 3 weeks in a 12 week cycle (i.e. 5 times doing back to back hard days of 90 minutes over a 10-12 week period). I found that the 2nd day required a big mental effort which paid off the last 10 miles on raceday. Pace on those back to back days was target marathon pace minus 15 seconds/mile.
Based on my results at other distances, I don't think I left a lot on the table in terms of my marathon pr. Maybe with a more optimal/modern training plan I could have gone 2:51 or so. But for the workouts I did I think I got a lot out of my racing.
In your case, January marathon and you seem open to doing long runs up through say mid-November. That's probably about 2 months before marathon day, right? If that's the case, get some long runs in during Sept-Oct-Nov. that will put those miles in the bank. Then when the weather gets colder and you don't want to do 2+ hour runs outside, turn up the intensity. Unless you blow off the long runs in Sept-Oct-Nov I expect you'll be fine.