In general we prescribe mountain running by time and not distance. However we track both distance and vertical as well and try to adhere to the recommended rates of weekly increases in these as specified in our book Training for the Uphill Athlete. It’s not science but a 50 mile week with 15,000 vertical feet is going to feel very different than a 50 mile week with 3000 vertical feet. Only with experience (by tracking) will you be able to adjust the training load appropriately.
I hope this helps.