MechE here, are you me 5 years ago? My first job was incredibly easy, to the point that my roommate who worked in the same position only showed up for around 5 hours most days and still did fine. I read all of MMM, and basically got to the bottom of the internet while working there. Technical knowledge was basically unnecessary, it was all paperwork and meetings. It was great for a little while, then it was incredibly boring, and like you I thought my skills would all decay because I hadn't actually done anything worth doing.
I've since moved to a more technical role in another industry, and it turns out I learned more from that job than I thought. How things get done in a large company. How/why bureaucracy is a thing. How to get people to go to meetings, how to be more efficient when presenting information, how to manage a project. I suspect you'll be surprised at how much you learn while at this job for a little while compared to someone straight out of school.
As someone above said, most jobs are pretty easy. My current job is pretty technical, but the reality is it's still as easy as my freshman level (or below) classes. F=PA, the burst equation, and A=Pi*r^2 are really the only "equations" I typically use, along with some basic addition/subtraction for tolerance studies. Most of that stuff is in excel sheets anyway, so it's very rare I actually have to do any math myself. Working in the real world is about far more than technical skills, although I suppose some research positions are probably more like school. The reality is, most companies figure out how to do something, and become specialists at that one thing. Sure maybe some calculations are sort of tricky the first time or two, but after designing a certain type of thing 100 times, the calculations are just busy work.
Learning how people work is much more important than calculus at the vast majority of jobs. Even design based ones require you to know how people are going to actually use your designs. Generally you don't need to know the specifics of high level classes, just the basics of how things work is plenty. What stress is, basics of how friction works, what lever arms do. You get to re-learn anything you actually need once you're on the job, you'll be surprised what comes back to you after a few lessons. School showed your employers that you have a capacity for learning mechanical skills. They're hiring people from lots of different backgrounds, they aren't expecting you to be an expert when you get there.
I'm glad I left my first job, but I don't think my 3 years there held me back. I learned about a different industry, and some of what I do now is easier because of it. I think 2-3 years is the sweet spot for a first job. Long enough to learn some things, and learn to work around difficulties at a job rather than just bailing immediately. Future companies will appreciate 2-3 years far more than 1 year on your resume as well. If you'd like more detail on my past/current job shoot me a PM.