"Races: In most of my training programs, I do not prescribe races. I don’t want runners feeling that they are obligated to race on a specific weekend, and at specific distances, because that’s what the schedule says. But a certain amount of racing is good, because it forces you to run at peak speed and provides feedback related to your fitness level. If you know your 10-K time, for instance, you can use one popular formula and multiply that time in minutes by 4.66 and get an estimate of your marathon potential. If you run other distances, you can use various prediction calculators to do the same. My favorite calculator is that offered on McMillanRunning.com. I suggest doing a half marathon in Week 9 to help you assess your fitness, but there is no magic to that distance or that week for racing. As an advanced runner, you should know how to make at least minor modifications to suit your schedule."
https://www.halhigdon.com/training-programs/marathon-training/advanced-1-marathon/#training-schedule
When Hal Higdon schedules a "Half Marathon" on the schedule, he intends for you to race it. I'd say if you want to race it you can, but don't feel obligated as the instructions say. In this particular plan (no idea if it's the one you are using), you can see he does very little taper or recovery from the race in Week 9. In fact, it falls in a "down" week in the plan with respect to the weekend long run. So depending on your fitness gains, you may or may not be able to see a PR level effort on that day. But then it may also serve as a good "end of race" marathon training run at a solid HM PR type effort.
There are certainly pros and cons that can be weighed in choosing to race a "B" race during an "A" training cycle. When I write training plans for others, I try to balance when those races occur in relation to the rest of the training plan. Far enough from race day not to impose additional fatigue (minimum is 8 weeks for me), doesn't come on a week where we were already planning a very important recovery week (typically follow a 3+1 schedule), how much of the "A" goal is a stretch goal vs current fitness (dictates whether we need to maximize training or adding in some recovery from a hard "B" race can be overcome), etc. Those aspects help me decide whether to give a green light on a certain race or not. The tricky part is knowing the runner and whether they can do a race as a "training run". Some get caught up in the day, some have a Type A personality, etc. that dictates that I shy away from agreeing to a "Training run race" on the calendar. But at the end of the day, I can always adjust the schedule when things happen.
