Are long sentences inherently bad, or what?

[paranoid marvin]

I think it also helps to read a sentence out loud. If it's too long, it soon becomes apparent.

 

[tinkerdan]

This usually includes the word independent.

it means two clauses joined in an ungrammatical manner

...two independent clauses joined...
You can have a number of relevant dependent clauses in a sentence and easily avoid being a run-on.
Here is a bit that is helpful.

https://owl.purdue.edu/owl/general_writing/punctuation/independent_and_dependent_clauses/index.html

This includes how to properly punctuate a sentence that has two independent clauses.(Run-on)

 

[JunkMonkey]

Just make the sentences the length they need to be. As long as the content does what it needs to, and the sentence flows, what's the problem? Out of curiosity I just pulled out a book from my shelves. I deliberately picked an older book knowing that the Victorian and Edwardians loved long, complex, circumlocutory language. HG Well's The First Men on the Moon (the the narrator, by happy happenstance a writer unable to write). The first sentence is forty-eight words long. The next one five. On the next page is a three word sentence, 'I took risks.' which is all the length it needs.