This calculator is a convenient tool for generating a detailed race plan. The calculator has two unique features: 1] It provides the elapsed time for every mile so that you can monitor progress during your race. 2] It incorporates a "split" pacing feature [See the next paragraph for details].
Empirical studies have conclusively shown that virtually everyone performs best when they run races using a "Negative Split". "Negative Split" means the first half of the race is run slightly slower than the second half. Most folks do best with a 2% negative split. Obviously, hills must be taken into account. You'll notice there are positive splits listed in the drop-down "Split Factor" menu. This enables you use a positive split if the first half of the course is predominately down hill. Positive splits should only be used with great care.
A couple of notes about negative splits: Ideally, negative splits should be run with a gradual speed increase from the start to the finish. However, in practice, only the best of the elites can approach this ideal, they only need to concentrate for a little over two hours when running a marathon. The rest of us would need to do so for 3 or 4 hours. Plus, one of the nice things about running negative splits is that if you are not having a good day or perhaps you were a bit too optimistic picking your time goal, at the halfway point you can decide to simply finish the race at your first-half pace.
Dealing with the transition between the first-half pace and second-half pace: You should start to pick up the pace about midway in the mile just before the midpoint mile and be up to the second-half pace by about midway of the mile following the midpoint. The key is to be close to your elapsed times just before you hit the mile marker before the midpoint mile and the one following.
A note about the Split Factor calculations: The average pace, and not the split-factored pace, is shown for two special cases.  Fractional distances [e.g., ".21" part of 10K which is 6.21 miles.] And,  The middle mile in odd numbered distances [e.g., mile 3 in a 5 mile race].
"Time" column shows your elapsed time. You should keep as close to this as possible during your race.
"Pace" shows your mile split times. This is useful if you maintain mile splits on your watch. The value varies in accordance with your selected "Split Factor".
You may find interesting my articles on modern, scientific training methods Science Based Performance Training.