Weekly Training Log: More Mileage, But Is It Better Marathon Training?

One of the reasons I started building my own Garmin analysis system was to compare my current training with exactly the same week one year ago.
This week's comparison was particularly interesting.
At first glance, I trained considerably more this year.
2025 vs 2026
| | 2025 | 2026 |
|---|---:|---:|
| Runs | 5 | 6 |
| Distance | 70.8 km | 81.2 km |
| Duration | 329 min | 417 min |
| Mean HR | 130.4 bpm | 131.5 bpm |
| Long run | 26.1 km | None |
| Longest run | 26.1 km | 16.3 km |

My weekly mileage increased by about 15%, from 70.8 km to 81.2 km, while my average heart rate remained almost unchanged.
That sounds like progress.
But when I looked more closely, the story became more complicated.
Running Through Singapore
This week was unusual because part of it was spent traveling in Singapore.
Despite the heat and humidity, I managed three consecutive runs there:
16.3 km
13.3 km
11.1 km
The latter two were deliberately easy, at 5:37/km and 5:47/km. After returning to Japan, I added another easy run, a 13.1 km steady session, and finally a 400 m interval workout.

So although the total volume was higher than last year, much of that additional training time came from running slowly in hot conditions.
And I think that was the right decision.
Summer training should not be about forcing winter paces in completely different environmental conditions.
More Mileage Doesn't Necessarily Mean Better Marathon Training
The most interesting part of the comparison was actually what I did last year.
During the same week in 2025, I completed:
26.08 km at 4:28/km, average HR 135 bpm, Aerobic Training Effect 4.6.
This year, I ran more total kilometers and included more speed-oriented work.
But I didn't do a long run.
That raises an important question:
Which week was actually better for marathon development?
The answer probably depends on what we mean by "better."
This year provided more overall volume and more varied intensity.
Last year, however, contained a much stronger marathon-specific endurance stimulus.
My Current Weakness Is Not Speed
This distinction matters because of what I've learned from my recent marathons.
I can run relatively fast.
My threshold pace is around 3:40/km in good conditions, and I can handle faster interval work.
But that has not been the limiting factor in my marathon.
The real problem begins after 30 km.
The challenge is not producing speed.
It is preserving speed when the legs are already tired.
That makes a controlled 25–30 km long run particularly important for me.
The 26 km run I completed last year may therefore have been more valuable for marathon-specific durability than the extra weekly mileage I accumulated this year.
Still, I Consider This Week a Success
There is another side to the story.
This was a travel week in the middle of a hot Japanese summer, including several days in Singapore.
Under those circumstances, I still completed:
6 runs and 81.2 km.
I don't think adding a 25–30 km long run on top of that would necessarily have made the week better.
Sometimes successful training means knowing what not to add.
The goal was to maintain aerobic volume during travel, survive the heat without excessive fatigue, and return to quality training afterward.
From that perspective, the week worked very well.
A New Metric I Want to Follow
This comparison also gave me an idea.
Garmin gives me VO₂max estimates, lactate threshold, and race predictions.
But perhaps a more useful personal metric for marathon development is much simpler:
How fast can I run 25–30 km at approximately the same heart rate?
Last year's benchmark is:
26.1 km — 4:28/km — HR 135 bpm.
When cooler weather returns, I want to repeat similar long runs.
If I can eventually run the same distance at 4:20/km, then 4:15/km, while keeping my heart rate around the same level, that may tell me more about my marathon development than a small change in Garmin's predicted marathon time.
Takeaway of the Week
This week's comparison reminded me that training volume alone can be misleading.
2026: more mileage, more speed stimulus.
2025: less mileage, but a stronger marathon-specific long run.
Neither week is necessarily better in isolation.
The important question is whether each workout addresses the runner's actual weakness.
For me, the next step is becoming increasingly clear:
Keep the threshold speed, but build the durability to carry it through 42.195 km.
That will be the focus when the summer heat finally begins to fade.