M67.5 here with my 5 cents.
IMO, sub-20 is definitely doable for the OP with his impressive background and probably enough spare time for training. But quite unlikely in 11 months, perhaps in 2 years.
My personal experience, in short. Was running sporadically and in an unstructured way from 40 through 60, making not more than 15-20 miles a week. When the weight came to a
frighteningly high number (74+ kg with my height 170 cm only) I said “enough is enough” and decided to come back to competitive running. Was unable to complete 5K at my first attempt erroneously thinking that 4:30/km was a moderate pace (as it was in my youth). So, decided to play long and progress slowly (but surely), mostly because I was very injury prone, worked full time (and keep working in aerospace engineering) and did not have enough time for training. In a couple of months (T0+2) was able to make it at 23:36. Then the progress was as follows:
T0+6 months: 21:48 (weight – 72 kg)
T0+12 months (was not running in winter for 4.5 months and only XC skied): 20:55
(weight – 67 kg)
T0+17 months: 19:56 (weight – 65 kg)
Did not give up after that and at T0+5 years (being 65 y.o.) made it at 18:28 (weight 62), and 38:03/10K. Stayed at a plateau for about 1.5 years and then started declining ("father time" finally pressed me and also injuries accumulated). Now almost totally switched to trail running.
Again, good luck to the OP! The goal is reachable, but it is not easy.