Let's narrow it down to only pages with the exact phrase "horse riding"
in them. Do this by putting quotes around the phrase you want to search for. This narrows it down to 10,600,000. Let's add vacation to the search terms. Since we don't need the exact phrase "horse riding vacation," type it as "horse riding" vacation.
This is very promising. We're down to 1,420,000 and the first page of results all seem to be about horse riding vacations.
Similarly, if you had results you wanted to exclude, you could use a minus sign, so horse -breeding would yield results of horse without the word breeding on the page. Make sure you put a space before the minus sign and no space between the minus sign and the word or phrase you wish to exclude.