In my opinion you are riding to slow or to rich on the main jetting if it looks like this after a 5 hour running.
The piston and cilinder head is completely black, there is no sign of any flow from the transfer ports over the piston head.
What is the color of the spark plug, and what is the heat rating of the plug?
The marks in the cilinder looks to me like it is from the piston coating, the place of the mark is above the bridge form the inlet of the cilinder.
At this point there is more pressure on the piston and coating.
No need to worry i would say, just keep an eye on the temperature that it is not to cold before hitting the throttle