I've had pretty good luck using this simple calculator to see some ballpark numbers

https://www.runnersworld.com/.../race-time-predictor

I realize it's basic and prone to error, but it worked pretty well for me---I ran a few halfs and then used its pace estimator to get an MP for training. I then validated those numbers by doing some training runs (like a 10 mile) to see how things compared. However it calculates things--it assumes that you've got a fair amount of miles under your belt and you've got things figured out well enough that you don't bonk.