How do you handle a physically demanding job? I'm pretty weak in general and I'm struggling at my new job?

I got this new position so, I can move up but I'm struggling with doing the easy task on this job. Which is unfortunate because I feel like I can't do my job right and I feel like they will let me go if I don't learn quick.

How do you handle a physically demanding job? I'm pretty weak in general and I'm struggling at my new job?
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