How much change are we talking about?

I'd try tracking your intake and exercise calories for a week...and see where things really are.

I'm running 40mpm training for a 10k. If you're coming from tri training, running ONLY (5-8 hrs a week)...won't burn nearly as many calories as SBR for 12-15 hrs.

How much is it actually changing? When are you weighing yourself?

Unless you've managed to find a way to defy the laws of physics. Either it's water weight or you're eating more than you realize.

1) The clock is the only factor on race day - you don't get penalty points per lb/kilo
2) Muscle weighs more than fat for same volume and is a lot more useful
3) 4 weeks is not any real time for any adaption. Both ways, you won't actually put any significant weight n 4 weeks unless you stop all exercise and just eat lard, equally, you won't really lose anything but water in the first couple of weeks of training.

So just don't sweat it. When you start doing 6 hour rides and 3 hour runs on consecutive days unless you refuel with chocolate bars and McD thinkshakes your body composition will morph to your 'athletic'. May be you'll skinny up, may be you'll build muscle. But either way you'll get quick.

If you want to ditch Ironman and just go for becoming 'buff' there's loads of 'better' ways to achieve that.

Or you’ll just look like an anorexic.

Yep. There's some autoregulation that occurs with appetite that is subtle enough to not appreciate. Unless the OP is meticulously weighing and measuring food to track, that's the likely variable that isn't accounted for here.

If you are significantly under fuelled your body might go into starvation mode and try and conserve calories.
So if you eat more you might lose weight.

Counter intuitive I know.

Haven't done a marathon in a few years but this happened to me every time.

Going from a weekly mix of swim, bike, run at various intensities and distances to just running and lots of it being slow can do that.

THAT is not a thing.

THAT is a thing. It may not be the starvation thing, but gaining weight from training when in theory you should be losing it is a thing.

:-)

That's why I highlighted the starvation mode. Starvation mode is not a thing... Or rather the effect is very minor... ~100 cal per day.

It is not a thing that violates energy balance, but it is a thing in your brain that commands you to EAT! and to conserve energy.

You're not accurately counting calories. Start using My Fitness Pal. Your increased training volume is increasing your appetite and you're eating more. If it's not clean the result is weight gain. You're typically not going to gain weight running a caloric deficit, so it appears your numbers aren't adding up. With that much running this weight gain cant be solely water weight.

Things that i did were to stop eating earlier at night. Nothing past 8pm, preferably eating around 6pm. I'm also on a plant based diet so that helps. One snack i like for dessert after dinner are rolled oats with a half scoop of protein powder and fruit. Its very filling and fairly low calorie or a peanut butter sandwich on low cal (35/slice) whole wheat bread.

Yeah. Because you're eating over your maintenance regardless of cause.

blood volume increases during big endurance training phases. if your times are not slowing down, do not worry about it.

I gained 10lbs during my 2 weeks of peak training for my first 1/2 Ironman. I’ve lost it now and I’m doing my second this time around I’m maintaining weight. Endurance sports by there nature are cortisol raising. If you eat more than u burn u get fat and your body is in a highly stressed condition hormonally. Lifting weights helps and watching your food intake. Because we crave sugary high calorie foods when we really crank up the volume.

Physically impossible.

If more goes out than what you put in, you lose weight.
Same thing with cars and gas.

Keep an honest diary and check the calories.

I seem to always gain some weight when marathon training. I attribute it to eating more than I need to for the calories I burn. I switch my diet to more simple carbs from just plant based carbs because of the miles and I'm pretty sure I don't need to do it. It's in my head.

I have a marathon next week and will go back to my normal eating pattern a week afterwards. Expect to drop the 2lbs I gained as a result. Not sweating it.

I lost no weight for the first six to eight weeks and then lost 18 over six weeks after a long layoff. I think it was growing muscle mass outweighing the fat loss. Went from 196 to 180 lbs. Race weight target was 178. Lots of up and down as well as I went long, hydrated, etc. Don't sweat it yet.

Sometimes this is a sign of over-training or exhaustion.

Think most has been metioned...

Muscle inflammation -> swelling weight?
Muscle growth?
Increased blood volume and capillary building?
Recovery food high sugar or fat?
Snacks?
Not enough fat in diet leading to retention?
After workout beers or alcohol?
Less sleep can lead to weight gain?
Properly hydrated leading to more stored weight?
Craving salty food - > water retention weight?