If I workout, will this inherited trait (of being stronger) not be passed down to my offspring (Lamarck's theory on evolution)?
Indeed, whether you work out or not will not change how strong your offspring will be. This is exactly why Lamarck's was wrong.



I would note however that Behaviour can be heritable and Your behaviour can affect your kids environment

Behaviour can be heritable

The probability of you working out is affected by your genes and so if you have a genetic predisposition for willing to work out, this will be passed on.

Assuming there are genetic predisposition for liking to work out, the fact that you worked out would not increase the probability of your kid to work out via genetic inheritance but the fact that you have a genetic predisposition for liking to work out will increase the probability that your kid will have the same preference and will work out

By the way, if you want to learn more about the concept f heritability you should have a look at Why is a heritability coefficient not an index of how “genetic” something is?

Your behaviour can affect your kids environment

Working out won't affect your genes but it will likely affect the environment in which the kid will grow. Here are three examples


Teaching


If you eat healthy, you will probably teach your kids to eat healthy too.


Womb environment


Also, if you eat healthy and are a woman, you will likely give the right nutrients to your offspring while still in the womb (assuming you are a lady).


Epigenetics


Your environment will cause epigenetic changes (changes not in DNA but in the very close environment of the DNA such as typically histone tail methylation). These epigenetics modifications will be directly passed to the foetus.

More information

A post your question is almost a duplicate of...


Is it the case that all changes in phenotype during life are not inheritable?


A post on heritability


Why is a heritability coefficient not an index of how “genetic” something is?


And finally an answer that talks about non-genetic inheritance


Why does evolution only involve heritable traits?