Marathon Training Plan - No Long Runs

On a HM program with race day May 17 the DSW had me on 1:20 minutes long run last week. The 8020 plan I switched to had about the same. 45 minutes sounds wrong!

Do you have the correct max heart rate / lactate threshold? I got the impression that suggestions are based off that.

My longest long run for the HM was about 2 hours. At least for the HM it seemed it did not what to push me beyond that.

My best HM is 2:10 so far (71 years old).

Superb time well done. My max heart rate is currently at 183 bpm. I am 42 years old. Does that sound right? 

Neither max heart rate nor LT should not make much difference for run duration, but rather for pace of runs. Like most of the features in Garmin world this one too works in semi random fashion and you should just suck it up.

Probably a good idea to give some background on your training.  I am assuming that the plan references your current Weekly Training Load, vo2max, and general typical training.... to then Build from.....   
What did a typical week look like for you before the plan was started?

What has the progression looked like from the early weeks until now?? 

How long in duration/distance are your easy runs and workout runs?

Agree with MatsR - all of the Garmin systems utilize HR data directly (vo2max and recovery time)....or indirectly (training load, training status, etc).  So your HR should be MANUALLY SET... by YOU.... based on all out intervals (or end of a race sprint...) gasping for life, blurry vision...etc!  OR (take a really hard interfval day with good HR data (tracks effort and looks "good") and ADD 5-10beats per minute because it wasn't all out).  Can sort activities by max HR from last couple years... then look at them for ones that were hard workouts and races.  Check HR for quality readings.

Thank you Nick. My training started first week of January (for race day May 4th). Before that I did 2 x HMs at the beginning and end of November 2024 with respective finish times of 1:42 and 1:38. A typical week for the past c3 weeks totals about 30km all in. 5 running days per week - 4 of 5 days will be Base runs at easy pace (5:30 per km) most frequently (3 times per week) these are 30 min in duration with one 45 min Base run (8.5km distance) plus one threshold pace run which consists of 10 mins warm up easy (5:30 per km), 15 mins at 4:20 per km, followed by 10 min warm down easy pace. My VO2Max is currently 52 which is indicated as 'Excellent'. The plan I'm on is targeting a finish time of 3:30hrs. All help very welcome. 

Did you continue to run in December?

You plan doesn't seem to be changing or increasing like it should for a May 4th race.... I would abort following that today. 

With roughly 7 weeks of build in training, and then 2 weeks of taper.  Really recommend just making your own spreadhseet 

One... start with your longest long run 2 weeks before race (april 19th)... figure out with the weeks remaining, how to progress from your current longrun (or one yyou think you could do pretty well, like "i know I could go do an easy 10k, recover well, so then start at 12k" )

Also I would try to work up to a midweek 'medium long run' trying to build that to 80-90min, generally accepted "plateau for benefits vs fatigue".  During build up of all miles, make sure your alternating difficult runs (workouts or longer duration) with super easy runs (very conversational pace, feel silly slow even!)

Thank you Nick - having done a bit of research just now on free plans online it does look like my long runs should be in the 27km rang and a total weekly mileage of c.50km at this stage. This is a huge step up in mileage from what I'm doing now. There is clearly something wrong with the Garmin plan. Very disappointing as I found the HM plans were excellent. Perhaps someone from Garmin might see this post (moderator please!) and can comment. Appreciate the advice Nick.  

Sad to hear that you need a hugh step up in weekly milage.

Please make sure you don't do this in 1 week, but build up slowly, as there is a serious injury risk with that.

4th of May means you have at least a few weeks to build.

Better undertrained at the start than not at all!

Good luck with your marathon!

I have made the same mistake, used Garmin plans before very successfully. They used to have 3 different coaches, 2 male and 1 female and you could pick them according to your preferences. 
They have got rid of them and I have found it keeps adjusting my runs according to my sleep, no way of turning it off. If it calculates that you have not had enough sleep it reduces your running lengths. I work shifts including nights so the watch has been incapable of adjusting for this. 
I am 4 weeks away from a marathon on a 12 week plan and longest run was 1 hour so far !