With the Fastsample sketch I got 80khz using prescaling.
So that is a sample speed of 12.5uS and you want to see the rise time of a 10uS pulse! That is not going to happen.
In my tests I used the PWM output on pin 9 with a value of 1. I was using an expensive scope > $8000 with a good probe, and I didn't notice any significant rise time on the pulse. You have to watch cheap scopes and probes they can add significantly to the apparent rise time of a pulse.