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. You'll have trouble following up on the letter and you may not come across well. If you're a skilled writer, fine.