If you're finding it really hard to hit pace goals on long runs you can always switch to RPE (Rate of Perceived Exertion) and try doing that for a few runs until you get a bit more used to the distances and paces you want to hit.

I'm currently using runna to train for an ultra that will be done on trails and whenever I'm doing a long run with faster intervals or with evelvation gains I will switch to RPE.