You can create new variable to hold the last position of the previous enemy.
CGPoint positionOfLastEnemy;
when you create new enemy assign the position of created enemy to this variable:
positionOfLastEnemy = CGPointMake(xPostion, yPostion);
if you need to update the enemy position you can do it in your collision detection method.
After that just use the positionOfLastEnemy variable to calculate the distance you need.
The other way is to add variable to hold the last enemy
SKSpriteNode *lastEnemy;
in your createEnemy method save reference to your created enemy.
lastEnemy = enemy;
And if you need the distance of the last enemy just use
lastEnemy.position