I tested McMillan's chart with my real PRs from way back when. Very accurate. How accurate will it be now, as I am not training with the same intensity? Only time will tell, but it serves as a guide for both current and potential times. My 22:42 applied to a 4 M run indicates a 29:41 is possible. The distance is close enough to presume my training is correct; it isn't going from 100 meters to a marathon.
The formula cannot know race day conditions, or my fitness. On July 4, I will be fitter, but the course will be more challenging than on June 7. If the formula can show me anything for July 4, I can be pretty sure sub-28:00 for 4 M will not happen. A good race from me can turn out a 28:30, though. Maybe I will shoot for that (equivalent to a 21:47 5K).