Do you feel you could take close to five minutes off your half marathon PR in 12 weeks? This would be a similar goal. It's a big improvement, however it is doable if you are fairly new to running or at least new to some sort of structured training and are still seeing big jumps in progress. If you've been at it for a while then it may be a reach.
1:12 - 1:14 is my prediction depending on the course. A mid-1:12 time (but at least sub-1:13) should be the main goal, which would be a decent improvement relative to the HM.