At the start of each week, identify your most important runs. Every run should have a purpose, but some deserve a little more attention.
If this is your first marathon, prioritize your long run by choosing a time slot when you know you'll be able to get the miles in. For example, if the kids have a soccer tournament all weekend, a weekday morning or Sunday evening might make more sense for your long run.
If you're training to run a PR, make sure to schedule tempo runs and speed work. Ideally, you'll cross every run off your list, but if you run out of time, it's better to skip an easy three-miler than an important long run.