Is Kizaru really able to kick at the speed of light? While Kizaru does a physical attacks (such as kicking or punching) Does he, or atleast a part of his body transform back into his normal form ? Won't he hurt himself attacking at the speed of light ?  I don't quite understand the concept of the speed of light, but I think attacking with the speed of light like that he will break his own body. How is he able to do that?
I think this has something to do with his own physical ability in combination with his devil fruit. But over all I don't think the attacks have a big of a strain on his body to begin with.

His devil fruit the Pika Pika no Mi allows him to turn into light and manipulating light around him. As he can manipulate the light around him to reduce weight  he could use this fact in his kick to reduce the weight and make up with speed reducing the actual impact on his own body.

Second of all Kizaru is a highly trained admiral with amazing physical abilities


  As a Marine Admiral, Kizaru holds the title of the strongest fighter within the World Government and Marines. He possesses great physical strength, capable of holding down Whitebeard's bisento with just one foot, and is capable of fighting Rayleigh and Whitebeard equally despite their Haki abilities. Kizaru got struck by a "Haki"-wielding Marco, and experienced little damage from the physical blow. His powerscaling places him comparable to Akainu, who managed to deflect blows from Whitebeard with difficulty. source  


So he might just be able to sustain it with his training. Which would mean that in longer extending battles his power usage would go down or even not usable.

And my third point would be the fact that it is a Logia type of fruit. 


  A Logia user can be shattered, split, or otherwise separated into multiple parts, sometimes down to the size of dust, and reform without injury. 
  
  In addition to their ability to become their element, Logia users can generate limitless amounts of their element and control it at their whim, giving them fearsome attack abilities. source


The Logia type enhances a user/owner in a totally different scale to begin with. The huge enhancement made by the Logia fruit might already be enough to not sustain injury by his own ability due to the amazing regenerative abilities he has gained with it.