And if they aren't adjacent to start with? ie. starting 5' away, moving another 5' away, then 10' back to attack, (combined for 15' of move), before retreating?
Actually, I think if you needed a 5' movement to get to them, you could take a 5' movement parallel (or even towards) them, and another 5' movement that would put you in range, attack, then use the rest of your movement after that (for 20' of extra running).