With markdown especially with version controlled text, start a new line for each new sentence. You can start doing this right now. Don't burden yourself with the thoughts of single space vs double space. Simply add a space after the period and press enter to start a new line after each sentence.
Add an empty line to show that you are now in a new paragraph. My rationale is it will help version control software diff your text better. Thoughts?
In particular, it plays well with most revision control systems which are line-oriented.
That said: revision control which is semantically aware of textual structure and sentences would also be useful. And I much prefer composing with sentences separated by horizontal rather than vertical whitespace, though I suspect I could change that pattern. It would however mean I'd be changing practices based on specific composing contexts.
That said, I two-space.
Many people do recommend this (newline per sentence.) This comes up for example in the issue tracker of pandoc.
This is complete different as 2 spaces change the final representation, while newline-as-space in markdown source do not.
The one downside is precisely that long sentences become harder to read because they either don't wrap, resulting in very wide text that's thus hard to read, or else are visually wrapped oddly (very uneven line lengths, again making them hard to read).
I first learned to use a computer when I was almost in high school and I soon discovered the amazing world of project Gutenberg. Even in 2001, I still only had access to dial up Internet (and that was a huge privilege in my eyes being in the middle of nowhere). Therefore, I loved downloading plain text files of books. I didn't like reading them as such though. I don't remember exactly why but I loved the "justify" feature in a word processor and would copy paste all the text file contents into word and delete all the line breaks to make it "nice". I don't remember why I did it but I read quite a few books from project Gutenberg like this.
Coming back to the topic, I'd say I am ok with leaving out the training space like others suggested or even breaking down a single sentence to multiple lines where appropriate. For example, I could turn the previous sentence into three lines. First linee has just the words coming back to the topic. Second line has everything before the oe and the third has the rest. That is just one possibility.
One good thing about things like markdown is that Only the writer and the editors have to fight with the uneven line lengths. The presentation is (at least hopefully) abstracted for the reader. Maybe we could even normalize the output text file from markdown as 80 columns. In this scenario, markdown is our source file that we "compile" into text or html as necessary.
To further illustrate my idea, here is the first paragraph from a tale of two cities [text file](https://www.gutenberg.org/files/98/98-0.txt)
It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way --in short, the period was so far like the present period, that some of its noisiest authorities insisted on its being received, for good or for evil, in the superlative degree of comparison only.
As a reader, you don't even have to think about where the line breaks are (: as an editor you can see "it was the" line up nicely for all those lines. I wish I could remember why I loved justify so much though...
[1]https://asciidoctor.org/docs/asciidoc-recommended-practices/...
It also lets you shuffle around sentences when editing which is nice.
Spent too many years in pine, I will type prose at 78 column width until the day I die.
Hopefully before that happens, change management tools will stop being completely moronic about diffing prose.
But whether that happens or not: one line per sentence?
Not a chance.
Unless I'm making a point.