I wouldn’t put any stock in your 1M, 10M or 1/2 mary PRs. Your 5k and 10k point to roughly a 3:20 marathon. Running 3:26 and being only 6 minutes from that isn’t too bad. I would guess that most runners marathon times don’t coincide (using a calculator) with their 5k and 10k times. If you’ve run between 60-70 mpw, you are on the right track. You’ve only been running 4 years, so I think you’re still relatively new to the sport. You still have lots of room for improvement. Do you run all year around? Or are you taking months off at a stretch due to injury or laziness? I don’t think there’s anything wrong with 1-2 weeks of down time 1-2 times per year after a hard season of racing. And you don’t have to be averaging 70 mpw every week. However, you have to avoid being inconsistent at various stages throughout the year. Consistency and patience!!!
I’m also thinking about a spring marathon and here’s what I’m focusing on this fall/winter.
1) Building my base back up. Like you said, as many miles as my body will allow. I’m not going crazy and jumping from 60 to 100, rather I’m trying to focus on what’s worked in the past, but doing it for a longer timeframe. For me, that’s 70ish mpw. I’ve pushed 80-85 before, but always seem to get hurt. So I’ll pay close attention if I get up that high again. Also I’m in no hurry. If I have to stay at 70 for 3-4 weeks before bumping it up, that’s alright. I’m also building/maintaining for 3 weeks, then dropping down for 1 week.
2) I’m doing some type of up-tempo work (tempo, cruise intervals, MP, etc) at least once every week or two.
3) Strides. I throw in 4-8 x 20 second strides 2-3 times per week.
That’s about it. Once February rolls around, I’ll start looking at a specific plan of attack.