All else being equal, which it never is, multiplying your 1/2 time by two and adding 10 percent is a good predictor. Working with absolute numbers (e.g., "add 10 minutes") isn't helpful because it suggests that 1:00 and 2:10, 1:40 and 3:30, and 2:00 and 4:10 are all equivalent pairs, and a cursory examination shows that only the middle pair qualifies.
I'd look at an untapered 1:17 as suggestive of a 2:37 if the longer stuff is in place, the weather cooperates, you don't screw up the pacing, etc.