The course lasted six weeks and met for two hours each week. Lots of work for just one letter. I mailed the letter to about 30 publishers and got 13 responses. That book was the forerunner of this one. That's how many of us feel.
Writing can be hard work.
Don't take it lightly, and don't feel bad if you can't write a high-impact marketing letter in half an hour. Writing is a profession, like rocket science. Don't expect to learn or perfect it overnight. I've found that people who copy someone else's letter seldom get a good response, regardless of how good the letter is. That would be quick, but probably not effective.