"In theory, 1:20 half marathon should get you below 2:50. But your raw 5K speed is a bit slow for the goals. How much do you weigh? Your 18 min suggests that you should not break 2:56. How fast can you run a mile? You might be one of those guys with a big engine whose run technique does not match the engine. This shows up at the mile to 5K range where you don’t have the timing and push off force and gait to run faster, and the same lack of run technique at the faster speeds, plays out again at the tail end of the marathon where when you break down, things go to shit. At the half marathon, you’re not at the top end speed for this to show up nor at the the limit of the endurance for the form/technique to limit you…just a hunch…and the bigger you are, the more it will show up at the tail end of the marathon. I’ve seen a number of bigger guys who can run moderately fast half IM and IM run splits who have relatively poor open marathon speed.
If you want to run sub 2:50, then you have to do a lot of running mileage at sub 4 min per K. In reality, this is not far away from your 5K speed, which is around 3:40 per kilometer."
This analysis seems pretty good. I have spent time trying to improve run form (but it is still bad).
I am 6’1"- 167. I have a typical swimmers build. Long arms, broad shoulders, big feet, short legs, muscles on back.
I am not even sure that the problem is raw speed exactly.
I was always one of the fastest kids for sprints in PE class.
(Not varsity track and field fast, but not slow either).
The problem definitely is in the 800m to 5k range.
I did do interval training for a number of years 800s, 1000s, 1200s.
This did not seem to work as well as the tempo runs I am currently doing.
There was short time where I tried to both tempo runs and intervals in the same week. (This was perhaps too much training stress).
I think the advice to cut the long runs a little shorter, might free up some training stress to try to do two hard runs per week again.
Although you are light enough for your height, close to 170 lbs is still 40 lbs more to haul around the course compared to a 130 lbs 2:49 runner. You’ll see that for your IM run splits or half IM splits, guys your size will generally “under achieve” at the full open marathon unless you can shed pounds and do more running at 4 min per kilometer pace. Realistically if you’re going to run 4 min per K for a marathon, it needs to feel “easy enough” to crank it up “at will” for pretty well any run all week. Conversely you will also find that the 130 lbs fast open marathon runner will under achieve at the IM run because of the penalty that tiny guy pays hauling his bike around the course (same size wheels is a bigger percent of his top line watts)
Personally, I think your easiest path is shaving down 10 lbs and doing more speed work rather than a lot more run volume…bigger guys with swim backgrounds with bigger engines just end up getting injured. You won’t get injured doing IM run training because the pace is slower, but if you want to get to pointy end marathon speed, then you need to comfortably run at those speeds often. I would add more plyometrics and skipping.
You mentioned that at shorter sprints you were plenty fast but at 800m to 5000m you fall off. This is because at 400m down, there is a big fast twitch fiber contribution to that speed. Your slow twitch fibers need to be able to take over and provide that good technique and elasticity over the aerobic ranges. It seems they can’t apply enough sustained force for your size, so either you make them more efficient through adaptation, or you lose weight (ideally you do both). Your aerobic engine from swimming is your friend, but at the high end of speed or the high end of endurance, your slow twitch muscle fiber just can’t do the work yet for your weight. Over the internet, this is my 2 cents. Your tempo runs need to have a lot of “over 4 min K and under 4 min K”…the old style fartlek might be your friend.
PS. You have the perfect build for Ironman and half IM, so depending on how important this marathon is, I would not mess around too much…have a look at Kona top 15 the last few years…pretty well all centered around 6 feet 160 lbs. Unfortunately, this is actually not the perfect build for open marathons…if it was all the Kenyans winning Rotterdam and Berlin would be that big, but they are generally 25-40 lbs smaller.