VO2 Max and race pace disparity

I have a vo2max of 54, and I'd say I'm almost 20% off from the race time my fenix 5 suggest, I mean I'm about 15-20% slower. I always thought they were too optimistic. I'd suggest to use some other race predictor like Tanda or similar, much more accurate.

Also, as other said, vo2max it's not necessary the best indicator on how fast you are, there are multiple other things, like running economy, your type training, the weekly distance you run, how you recovery, how you eat, how you breathe, and, especially if you are a beginner, how much you can push (usually not too hard, it's just the way it is).

That said, use garmin vo2max as an indicator of the progress of your training. If it increases you'll see your race time get better, otherwise you'll be slower. But again, do not predict you pb just looking at your vo2max.

One very generic tip I can give you for your marathon training is, increase your weekly distance a lot, 35km/week it's the bare minimum to complete a marathon IMHO. Start from 35, then increase gradually maybe to 70-80/week, more if you can. Running 5k blocks is useless for endurance, do long runs (15-20Km) and incorporate some speed work. Also always keep a good running form.

Exactly this.

Great suggestions on increasing the total weekly run. I used to think 40k would be good for it. It was for my first marathon. Took 5 hrs though. My Half is 1:57 though. this year I'm taking your approach and working on longer runs with speed workouts in between. Hope the world returns to a normal this year and the London Marathon happens, I'm training for a 4hr30min goal, and it looks achievable based on my comfort with the long runs @6:25 so far.