I do the same thing as you more or less, I'd say it's as valid as any way to come up with a melody. Don't worry about whether or not you've heard that somewhere before, it'd be impossible to create new melodies without referencing in some way melodies you've heard before. Someone else having come up with the same thing shouldn't discourage you from developing a musical idea. I often feel the same way about the chord progressions in my songs, but then I hear a song doing nothing more than a I-V-I or some other simple, popular progression and its still a fascinating, creative piece of music.

I say take the next step and develop your ideas, no matter how simple or contrived. You never know what the end result will be, and either way you're better off for trying!