The real question for guys like you is: are there any differences in approach and structure between vocal songs and instrumental songs. Answer: yes, and they mostly have to do with the strengths and limitations of various instruments (of which the human voice is one). Let's check them out.
First off, the human voice is a monophonic instrument (Tibetan monks notwithstanding). It is incapable of performing a harmonic piece the way a single guitar or piano might. Obviously, a composer would need to take that into account.
Furthermore, instruments such as electric guitar and keyboard have a much wider range than the human voice. Melodies composed for them can take advantage of that. There are also musical passages idiosyncratic to various instruments that are impossible to sing - phrases that are too quick or which contain gigantic interval leaps, for example. Lastly, there are often quirky noises or instrument characteristics (pick slides, pinch harmonics, whammy bar dive bombs, e.g.) that can define instrumental melodies, that the voice cannot duplicate.
To better illustrate the ideas above, I challenge you to sing the lead guitar part to "Cliffs of Dover" by Eric Johnson. Let me know how it goes.
Form and Structure
Structurally there can be differences too. It really depends on what we are including within our definition of "instrumental". If we restrict our definition to short, contemporary "songs" without vocals, I would say there doesn't have to be a difference at all. Verses, choruses, bridges - all the standard building blocks of songs - can be manipulated and ordered just the same in both cases.
The one big caveat I can think of is the hook. In vocal songs the power of the hook is strengthened and reinforced by lyrical rhymes. Without the benefit of rhyme, instrumental hooks have to be musically memorable, which is why they often depend on instrumental idiosyncrasies, as mentioned above.
The bottom line is: write to put your melody in its best light. Take advantage of the capabilities of your lead instrument, whether it is the human voice, a saxophone, or a screaming guitar.
*Photo by eclecticlibrarian.