consider volume, and workout type
Your calculation should look at running volume and workout type, but there is no way for you to no the effort needed to achieve the volume. Your pace calculation assumes I am racing every single run I do. I might for example run a fast 4 miles inside a 9 mile run but with 5miles kinda slow, the overall result would be slowish, and would not reflect my real speed potential. If I follow a quality run schedule with 3 very high quality runs with lots of rest, your tables look very accurate, but if I mix it up with lots of junk miles to increase my volume the predictions are much, much lower. None the less I love this app you created and I also like you are asking for ways to improve/refine. Another thought is I do slow long runs in the 20-23 mile range, and fast runs in the 12-15 mile range, but the 20 miler runs I think are making me faster even when I run them 2-3 min per mile slower than race pace.