It's Ray Haffner! When Sophie mentions the excellent whistler they take our attention off Haffner by showing McAllister. Remember Haffner whistling at the hosital...
Middle age: This rules out Stiles and Bertram in my opinion.
In good health, showing Haffner doing his workout is a hint of this for me here. 
I think of Kirkland as Red John's right hand man. 

All the signs are pointing to Haffner for me ;)