Time in zones is really the only important thing, mileage is just a product of that. If you bought some hypothetical magical shoes that made you run 10% faster, and you ran for an hour at an easy pace, the only difference would be that you covered 11 miles in those shoes but when you wear your normal ones you only cover 10. Either shoes, you're doing an hour at an easy pace.

As for X and B, if they're both spending those 10 hours at a similar load (% threshold or whatever), stimuli would be about the same. But the physiological changes that may occur from that load might be wildly different - B might be tapering and normally do 20 hours, and B might see a decline in fitness if they continued to run 10 hours, whereas if X is used to 7 they might see some real benefit.