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The RW Race-Time Predictor
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Sounds spot on to me TBH. If you did 19.5 in 3:16 thats a pace of just over 10:03. There are another 6.7 miles in a marathon and to do in 4:28, if you were to run at end of your 19.5 you'd have to run them in 1:12, or 10:44 pace.
If you ran your 19.5 for the best possible time you should be exhausted at end and certainly unable to run another 6.7 only 41 seconds a mile slower than your avrage over the previous 19.5...remember you hadn't even reached 20 miles and its usually after that most get into trouble (or as they approach it and realise they're struggling 8 miles from home).
It may be that you were fresh as a daisy at end of 19.5 and could have run the remainder at the same pace, but thats not comparing like with like, one run would be slow training run, thenother (marathon) you'd have pushed on relativley harder. No calculator can account for that. Also you'd run the longer distance (if you had sense) slower from the start rather than slowing down like that.
But beware that last 10k, is a killer and those who undrestimate it end up in trouble.
FWIW I've found this calculator spot on for me.
If it was just a training run then it's like comparing apples and oranges.
Also, if it was a training run, are you running then at target marathon pace?
https://www.runningahead.com/tools/calculators/race
It works bang on for me. From 5K to HM, I am routinely within 10 seconds of the predicted time from a recent race. If I have a good marathon, I'll be within 30 seconds of the prediction.