I'm a little quicker than 1:20 (recent 1:15), but you can extrapolate times to suit. This is what I did since January 2004 to get to this recent HM result (not that the HM was a goal last January, but this is how the training evolved as life happened):
All miles are "easy" except the work sessions or races, as described. "Easy" generally means 125 bpm or lower, generally at 7:00 to 8:00 (mostly around 7:30, plus or minus, but often slower than 8:00 or even 9:00). Aerobic work sessions are guided by HR, as shown. Intervals are run at prescribed pace, usually starting a little slower and trying to finish a little faster.
Initial aerobic base work, coming off knee injury:
Week 1: 30 miles, all easy, 9 mile long run
Week 2: 39 miles, all easy, 12.5 mile long run
Week 3: 62 incl 80 min @ 135 bpm and 61 min @ 136, 15 mile LR
Week 4: 63 incl 70 @ 138 and 55 @ 141, 16.5 mile LR
Week 5: 60 incl 65 min @ 140 and 64 @ 138, 15 mile LR
Week 6: 74 incl 3 x 20 @ 144 and 60 @ 139, 12 mile LR
Week 7: 63 incl 3 x 20 @ 145 and 60 @ 139, 14 mile LR
Week 8: 82 incl 3 x 20 @ 139, 8 s “all out” hill sprints, and 60 @ 140, 16.6 mile LR
Week 8: 77 incl 3 x 20 @ 144 and 60 continuous @ 140, 14 mile LR
Week 9: 85 incl 2 x 30 @ 144, 16.5 mile LR
Week 10: 77 incl 3 x 12 @ 148, 15.5 mile LR
Week 11: 77 incl 60 min @ 144 during 19 mile LR
Switch to focus on some spring summer races:
Week 12: 57 incl 5 x “mile” @ 5:50ish and 60 min continuous @ 144, 13.5 mile LR
Week 13: 81 incl 4 x mile @ 6:00ish (faded badly) and 3 x 20 @ 144, 15 mile LR
Week 14: 53 incl 5 x mile @ 5:44ish and hilly, windy 5k race @ 17:09, 12.5 mile LR
Week 15: 54 incl hilly 10k race @ 35:17, 10 mile LR
Week 16: 66 incl 5 x 1600 @ 5:38ish, 4 x 1000 @ 3:18ish (faded, workouts too close), and 2 x 30 @ 144, 14.6 mile LR
Week 17: 81 incl 5 x 2000 @ 7:14ish, 5 x 1200 @ 3:58ish, and 75 min continuous @ 144 during 15.5 mile LR
Week 18: 72 incl 2 x (4 x 400) @ 72ish, 14 mile LR
Week 19: 66 incl 5 x 2000 @ 7:07ish, 16 mile LR
Week 20: 62 incl 1500 race (debut) @ 4:26 and 2 x 5000 @ 5:55, 13 mile LR
Week 21: 66 incl 4 x 1200 @ 3:55 and 4 x 2000 @ 6:52ish, 13 mile LR
Week 22: 50 incl 3 x 22 min @ 148 (fading pace) and 5k race @ 16:48 (felt like crap), 16.5 mile LR
Week 23: 53 incl 6400 m @ 144 (fading pace), 12 mile LR
Week 24: 60 incl 3 x 15 min @ 145 (fading pace) and 1500m race @ 4:25.7, 12 mile LR
Week 25: 71 incl road mile @ 4:30.8 and 2 x 20 min @ 145 (fading pace), 14 mile LR
Week 26: 64 incl 1500 race @ 4:18.7 and 3 x 20 @ 144 (pace steady again!!!), 14.5 mile LR
Week 27: 52 incl 5000 race @ 16:13 and 10k @ 35:30 (dead legs!! Too soon after 5k), 12 mile LR
Switch to focus on planned fall marathon:
Week 28: 66 incl 2 x 20 @ 144 (faded in heat) and 2 x 30 @ 6:33/138, 15.5 mile LR
Week 29: 80 incl 3 x 20 @ 6:23/144 and 40 @ 6:45/136, 14 mile LR
Week 30: 93 incl 3 x 20 @ 6:33/144 (v. hot) and 3 x 4800 @ 5:55/150, 18.5 mile LR
Week 31: 85 incl 75 @ 6:50/137 (v. hot/humid) and 2 x 8000 @ 6:09/150 (warm, humid, windy), 15 mile LR
Week 32: 67 incl 2 x 8000 @ 6:18/144 and 10k race @ 34:21, 14 mile LR
Week 33: 90 incl 3 x 3 miles @ 5:52/147 and 10 miles @ 6:18/139, 14 mile LR
Week 34: 55 incl 6 miles @ 6:02/149 (faded from 5:48 to 6:11) plus 5 mile race @ 26:42, 14 mile LR
Week 35: 65 miles, all easy, 13 mile LR
Week 36: 76 miles incl 4 x 5k @ 6:15 (147-152), 14.5 mile LR
Week 37: 79 miles incl 10k @ 6:00/150, 8 x 1200 @ 4:13-4:15, and 10 miles @ 6:14/143, 16.5 mile LR
Week 38: 63 miles incl session with 10k and 5k @ 6:03/151, 15.5 mile LR
Week 39: 45 miles incl 3 x 1200 @ 6:00 pace (light workout) and disastrous HM @ 1:18:58 (lungs full of snot), 14.5 mile LR
Week 40: 36 miles all easy, 11.5 mile LR
Week 41: 75 miles incl 2 x 5k 200/200 fartlek @ 37.3/55.6 and 36.5/55.6, and 8k plus 4.8k @ 6:03ish/150-152 (strong wind), 18 mile LR
Week 42: 73 miles incl 9.6k and 4.8k @ 6:03ish/150-152 (mild fade), 4 + 4 + 2 mile @ 6:17/144-146, 16.5 mile LR
Bailed on fall marathon plans due to nagging health/allergy/blood (??) troubles.
Switch to focus on fall races and winter indoor track:
Week 43: 54 miles incl 2 x 5.2k @ 5:58/150 and 8.2 k XC race (28 low – windy/lotsa turns), 12 mile LR
Week 44: 54 miles incl 8 x 1200 @ 4:15 w/200m jog rest, 2 x (6 x 500) @ 99s w/ 100m jog between reps, 700m jog between sets, 13 mile LR
Week 45: 52 miles incl 3 x 1200 on snow, 3000m race (9:31), 12 mile LR
Week 46: 67 miles incl 5k @ 145-150 (faded) and 4 x 500 @ 97s plus 6 x 1200 @ 4:11 with short rests, 13 mile LR
Week 47: 52 miles incl 10k XC @ 35:41 (very windy, a little slippery, whoulda worn spikes), 12 mile LR
Week 48: 71 miles incl 2 x 4.65 miles @ 6:05/148 and 5 x 1600 @ 5:31 w/70s (200m) jog rest, 15 mile LR
Week 49: 48 miles incl 8k + 5k @ 5:59/147 and 3000m race (9:17.7), 12.5 mile LR (missed 2 days achilles)
Week 50: 85 miles incl 2 x (5 x 400) @ 75 avg (planned 70-72 s - very slow/wet/soft track conditions), 1 x 3200 and 2 x 1600 @ 85-86s/400 pace w/ 200m jog rest, 12.5 mile LR
Week 51: 77 miles incl 6 x 1000 @ 3:20 avg (very windy, slippery roads) w/200m jog rest, 5 x 2000 @ 6:47avg w/200m jog rest, 14.5 mile LR
Week 52: 60 miles incl 3 x 1200 @ 4:08 w/200m jog rest and 3000m race (9:18.4) plus 14 mile LR
Week 53: 81 miles incl 8 x 400 @ 68s-low plus 5k @ 6:06ish, also 2 x 2 mile and 2 x 1 mile @ 5:40ish plus 3 mile @ 6:53/147 (doh!), 15 mile LR
Week 54: 47 miles incl 4 x 400 @ 74s w/200m jog plus 2 x 2000 @ 7:00, 11 mile LR
Change focus to HM:
Week 55: 83 miles incl 6 x 1600 @ 5:32 avg w/2 min jog rest and 2 x 3 mile @ 5:39 avg w/800 m jog rest, 17.5 mile LR
Week 56: 61 miles incl 4.3 miles plus 3 miles @ 6:00/150 and indoor 1500 race (4:22.5), 13 mile LR
Week 57: 83 miles incl 2 x 5 miles @ 6:15/143, 2 x 3 miles @ 5:48 plus 5k @ 6:28/140, 15 mile LR
Week 58: 57 miles incl 4 x 2400 @ 8:20 (5:32-3 pace) avg, 3000m race (9:22.8), 12 mile LR
Week 59: 65 miles incl 2 x 5 miles @ 150ish (faded badly) and 2 x 5k @ 150ish (v. strong winds), 15 mile LR
Week 60: 55 miles incl 3 x 2 miles @ 150ish, and 2 x 3 miles @ 5:37 avg w/1:38 jog rest, 13 mile LR
Week 61: 50 miles incl 3 x 4 min @ HM effort and HM race (1:15:03), 15.5 mile LR
Week 62: 59 miles all easy, 12 mile LR