So, you want to feed it up, get it longer -- and somehow or other not turn it into a padded monstrosity. (Conversely, you may want to shorten it. In which case you can look for advice elsewhere, since I've never succeed at that. 0:)
Assuming you got the aesthetically correct length for that story, it's tricky. You have to transform your story so that it is complete and aesthetically sound at a different length. The longer story needs a different structure.
1. Consider where your hero got help, and see if you can make the source of the help unfriendly, hostile, or apathetic up front. Transform it into a threshold guardian. This is one of the best because it improves the story by adding conflict and difficulty. However, the wordage is not great, and it may make the story episodic.
2. Promote characters. Your main character is still your main character. But the major characters, the minor characters, the bit characters -- look for their stories. Particularly useful if you can make the events of the story as important to them as to the main character. Give them internal conflicts. Develop their motives and their character -- well, development. Give them story arcs.
It is very likely that you will have to introduce more POV characters to get it all in. This will broaden your story, and by the same token diffuse it; it may improve it, it may weaken it, but it definitely will change it. The characters may also demand more scenes, before and after the ones where they appear at first, particularly if they were only in the same locale as the hero for a brief time.
3. Introduce new elements. A character, or characters forming a new faction with a new aim. Not just a plot twist, because it has to reverberate through the story. It has to interweave through the existing elements so that it is an integral part and affects the old stuff, while the old stuff affects it, and have its own arc and motivations and all that fun stuff.