This is a little difficult to explain, especially without appearing crass or it coming across as a humble brag or something like that.

Let's just say in the USA there's a strong culture of denial and reinforcement surrounding being unfit and/or overweight. If you live long enough surrounded by that environment, you start believing it yourself and being part of it.

So when I started making changes without any specific goal of changing my appearance, I was just trying to make myself stay youthful longer for quality of life reasons, the appearance stuff was a necessary side effect. It was pretty jarring to have women agressively flirting with me since it was completely unfamiliar territory, nor part of my goals, and a lot of those lies this culture surrounds what are arguably unwell people with just kind of shattered: yes looks matter, yes I was previously unattractive, yes people are shallow, no your girlfriend saying weight doesn't matter on guys is not being honest, etc.

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One of my friends married a Chinese woman, and I was visiting where they live (now in the US) for a couple months. We hung out when I first arrived, and I was in really excellent shape with well-fitting clothes. Just in the course of a couple months spending time in a different area, with a drastically different (unhealthy) food culture, and generally taking less care of myself, I must have put on 10-15 lbs.

The next time we hung out, the first thing that came out of his wife's mouth when she saw me was "you're getting fat!"

Americans don't do this, and it's a disservice to everyone. I was not bothered, and it made me laugh, knowing it was just a temporary condition. But this is damn important feedback for people to hear rather than letting everyone exist in total denial enabling further self-neglect.

I've had a number of experiences like this with Chinese women, their culture is a lot different in this regard. Americans tend to see it as rude, I see it as necessary, and really appreciate the honest unsolicited feedback.
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There was just some adjustment that needed to take place. It's learning you've been lied to for a very long time by many people to the point you were lying to yourself. Kind of similar to the atheist rage that commonly occurs when someone raised in some religion realizes everyone growing up was lying to them the moment they discovered it's all fiction.

Then there's the challenge of simply acclimating to having all this attention from the opposite sex when previously there was none. Depending on your personality, this can be quite a burden and stressful. I miss the freedom to just go out and be ignored everywhere I go. It's not something I predicted losing when I started taking better care of myself. But when you're fit, and I'm not even talking like some body builder or anything - unfortunately in the USA it doesn't take very much effort to be well above the average fitness - people give you substantially more attention.

I'm nothing special. All it seems to take is doing a few hundred pushups every day and eating copious amounts of produce, for years. People get all hung up on spending hours exercising, seriously just spend a dozen minutes a day on lots of pushups every damn day and you'll get most of the benefits exercise has to offer in my experience.

But be warned, socially, I've found it's a mixed bag.