The obvious question is, what is the course for your marathon like? If it has hills, you need to find hills to train on. If it's as flat as where you can easily train, lack of hill work shouldn't be a problem. Run as many of your training miles as you can manage on terrain similar to what you'll see on race day.

When avoiding injury is the priority, I would replace the "AYF" runs with "easy," meaning, "don't run fast." Save the fast stuff for the designated tempo and interval runs. With no hills to train on, substitute intervals for hills but perhaps don't make the intervals as challenging as the days when intervals are really assigned.

That schedule has a good chance of taking you from 20 miles per week to being able to finish a marathon. Expect to be really wiped out at the end of the marathon, and expect to need substantial recovery time after the marathon. This might mean one to four days of no running after the marathon, and a full month of running only easy pace after the race. More people get messed up working too hard before fully recovered from a marathon than get messed up from actually running the marathon.

Given the low weekly mileage of this training plan, I would be very reluctant to take a flex day and swap it out for something else. If you get sick or hurt during the training cycle, that's a different story; if you're sick enough to miss a run, don't try to make it up. Every training plan in the world assumes you will stay healthy for the entire training cycle. In the real world, the first training challenge most beginning marathon runners face is how to adapt the plan when injury or illness strikes during the training cycle.