How much can you realistically improve in a year?

I'm in my 40s and have been running for 20 years. I just got serious about training in the past few years and ran my first marathon a week ago with a time of 3:58. I think I could've done better, but I had ran a half-marathon 3 weeks ago when I really should have been focusing more on the 18 - 20 mile length runs.

During the marathon, I was averaging 8:30/mile, but by mile 22 I had hit the wall and had slowed to a 10:00/mile pace. I ended up walking bits of the last few miles because my legs were toast. I just didn't have the endurance I thought I would have in the marathon (my goal was 3:48) towards the end.

In October 2024, I'd like to aim for 3:20 in the marathon so that I can qualify for Chicago in 2025 by time. Is it feasible to improve from 3:58 to 3:20 in a year, or is this out of reach? I'm willing to work hard and be consistent, but I also I don't want to set myself up for disappointment with unrealistic goals.

More broadly, are there guidelines for how much you can improve year over year?