I was probably seventy-five to a hundred feet from each of the subjects.
With the mirror 500mm, it's easy to be stealthy. These individuals had no idea I was photographing them. The lens itself isn't any longer than my 24-70mm. Couldn't be anywhere near as inconspicuous with my next longest FL lens (70-200 f/2.8 VR), which is twice the physical length of the mirror 500.
With the fixed focal length and f-stop, it's best to have the subject be in direct lighting to get the fastest shutter speeds. I cranked the ISO to 1000 yesterday to help with that. 500 would have gotten me what I needed, but I actually wanted to have the extra noise. And, with that ISO in those lighting conditions, my shutter speeds were routinely in the 1/3200th range, so there was zero worry about camera shake blurr. Any softness to the images I got was related to the amount of attention I paid to ensuring perfect focus.