If you had been running an hour and a half a day at a comfortable pace and switched to an hour you might run slower. But if you were worn down because the hour and a half was more than you could handle at the moment you might freshen up and run faster for a while. If you were doing some days at or near your 5 mile pace and stopped doing that you and just ran comfortably for an hour you might slow down. But if you were overdoing the near race pace stuff you might get faster. If you'd been running for half an hour a day comfortably I suspect you'd get faster at least once you've adjusted to the higher work load. But if you maintain the same pace, even when you're fit enough that something faster than 7:40 feels comfortable, you'll probably get stuck at some level.
Essentially there are too many variables in play to make any single sort of answer possible. I recall a quote from Nic Bideau to the effect of "If you run for an hour a day you'll get quite fit. If you add a run of 90-120 minutes to that routine you'll get fitter and if you add a second longer run to that you'll get fitter still." And here's another take on the idea:
It's geared to high school runners but he points out that the idea came from thinking about training adult runners.