Running a Half marathon under 90 minutes requires consistent training, maintaining the required pace for 13.1 miles @ 6m50s per mile pace required to run 90 minutes My best HM time is 85 minutes and that required holding 6m 30 secs mile pace for 13.1 miles. i have ran 10 Half Marathons, 4 times I have ran under 90 minutes including an 85 minute HM. I have ran 9 marathons, 6 marathons 3hours 20 mins or under with a best Marathon time of 3 hours and 4 minutes and I have also ran 3 hours and 5 minutes and 3 hours and 9 minutes. So close to breaking 3 hours but fading badly over the last 10 miles added 10 to 20+ minutes to my finish times.
If you are plodding 9 or 10 minute mile pace for a Half marathon to run around 2 hours, not much training is involved to achieve that goal. You can not expect to take 6 months off and run under 90 minutes in a HM unless you are a talented runner. Discipline, training and conditioning are required to maximise performance in distance running. You must stick to a strict diet, make sacrifices, minimise alcohol and keep weight down to maximise running performance.