Hide secrets in an article composed with Machine Learning?
To go for the low-hanging fruit, train your machine to be really good at swapping synonyms based on context.  Then just have it find suitable synonym replacements for every word of covertext located at a multiple of your magic number.

EDIT:  I don't have enough reputation to comment so I'll reply to another post here.  Someone objected that the keyspace is quite small.  For example, there are only a handful of possible keys in the OP's example, only one of which yields meaningful text.  But this can be remedied quite easily.  Instead of using simply the first letter of a word, use the letter at an arbitrary position determined by a second key, wrapping around if the word is too short.  

For instance, take the key KEY as numerals derived from the position of K, E, Y in the alphabet: 11 5 25.  Choose a magic number like 3, and beginning with the first word, hide a secret letter at position 11 (wrapping around if the word is shorter than 11 characters). In the 3rd word after (since 3 is your magic number) hide the next secret letter at the position indicated by the next numeral of your key, etc.


An apple is healthy to eat. They keep doctors away. You can cook them
in pockets of bread and enjoy!
[A]n apple is heal[t]hy to eat. [T]hey keep doctors aw[a]y. You can
[c]ook them in poc[k]ets of bread and enjoy!
Word 1, position 11: A

Word 3, position 5: T

Word 6, position 25: T

Word 9, position 11: A

Word 12, position 5: C

Word 15, position 25: K


This approach is not trivially broken.