Depends on how much you are willing to run and of you can stay injury free…
1.28 is a solid half!
I would argue that 12 week might be to short to train for sub 3, but you don’t know unless you try… I say go for it, you have absolutely nothing to lose.
I’d loose 12 weeks of my life 
Nah.
You will get a lot of fitness in those 12 weeks that can be use for cycling!
My experience is that the level and duration of soreness after a race is directly related to the volume of run before hand. 100 mi months, very sore. 150 mi months, kind sore. 200+ mi months, not that sore.
I think this 100% depends on your run volume so far and if you can build it to a reasonable level. 1:28 is on the slower end of someone doing 3:00 in the same training cycle (rule of thumb of M=2xHM + 10’). It does mean that the ability is there. If you keep consistent running and build some volume, a <3 marathon this fall or spring next year should be very doable, even if one 12wks from now may be out of reach.
I’d agree with that although I’d add 25 miles onto each of those 125mi, 175mi, 225mi it depends on the volume you are use to.
I’d also add it depends on your average training pace verses race pace, the closer your average training pace over the last 4 - 8 weeks to your race pace the less sore I would suggest you will be.
Broke the 2hr for a HM on Sunday on an easy’ish training trail run, only looked at time as I approached 20km… first time under 2 hours in probably 20+ years, but I don’t go all out ever, maybe try that one day on a proper HM event… but I’ll never get under 1’30 for a HM, kudos for that… I know I’m doing a lot of easy miles but enjoy them all, haven’t pulled up that Garmin stat before, this is mine… 
Do regular HM’s and never sore, but as stated I never go all out so probably not a good study…
I have done 100 miles in one week
I used to do about 300 per month… But that was before becoming a triathlete
For the marathon i did last February, i think the max i did in one week was maybe 45…
With that I was able to do 3.12 on a hot and windy day with 12 weeks of training
Same with the exception of my HM PB which was a hard but not 100% TT, in a race I think I’d knock a minute off.
Anyway most HMs 1:45 to 1:50 no soreness. PB 1:38:59 sore for 3 days.
You used to do 300 miles per week? 
Oddly, I know quite a few guys who have never run faster than 1:28:xx and have gone sub 3hr. Seems odd to me, but I chalk it up to them being diesels that can run one speed for a long time.
Month!
Jesus… dont post and drive children.
I’ve absolutely no reason to expect this is true for me though - if anything I would expect to currently sit the other side of the bell-curve with my low mileage to date.
Good to know that a sub 3 is a realistic aim if I were to train for it though.
Odd to me as well. 1:28:xx is just about the same pace as 2:59:xx, just for half as long (within day to day/race to race/course to course type of variations).
You have the bike fitness. You need to translate that into running fitness…
You MUST up your volume, sadly there is not way around it.
Your bike FTP might suffer some but not as much as you would expect.
I think if you can get your millage to high 40s low 50s… run 5 days a week, 1 speed day, 1 tempo and one long (maybe combine tempo and long)… and z2 cycle, for 16 to 18 weeks I have no doubt you can hit sub 3 on perfect weather marathon… with a similar schedule I was in shape for a 3:08 on the same course i did 3:12 on a with better weather (colder and less than 25-30 mph wind gusts going up a causeway). On a perfect course and day I think i had the fitness for a 3:05-3:06. That is with 14 weeks of training and only 1 20 mile run.
Interesting. Maybe they didnt train as hard or tried as hard on the half?
Stupid question, as far as you know, the 1:28 half, was it a stand alone race or was it during the marathon…
Some people use the half time as their PR if they have never raced a half by itself.
Reference bike fitness, I’ve dropped from 5 bike workouts to 2 a week (Threshold and Sweetspot) and now doing 3 runs a week. FTP has dropped by about 8-10%.
Heh… glad I’m taking it gradually then. My April plan is for 50 miles, and I’m trying to build up veeeeeery gradually to the Staten Island Half in NYC on October 8, then to the full NYC Marathon next year.
At least now I’m definitely seeing progression. My FTP just hit 213W (1.97 W/kg) which is my PB, I survived a 3x20 sweet-spot workout for the first time ever, and I’m now able to jog 5x weekly for a total of roughly 10-12 weekly miles.
I figured out that my issue is not excess weight, but rather that I’ve eaten so badly for so long that I’m already insulin-resistant. Changing will take much more effort, for a much longer time, than I thought it would… but as they say, direction is more important than velocity.
I assume those halfs weren’t in a race or a TT effort, otherwise it makes little sense.
I have seen the equation modified for fast / slow twitch, which is as posted the half doubled plus 10’ +/- 2’
I.e the extremes
FM = 2HM + 8’ or the other
FM = 2HM + 12’
Or you can try FM = 2.19 x HM (think I have remembered that multipler correctly.)
That is what you would see if you looked at my Half compared to my Marathon, I’ve never totally destroyed myself in a Half but have in a full.
