There is an interesting multipart article on learning to fly a Mustang here:
http://www.airbum.com/pireps/PirepMustangBurch.html
The author takes a "warbird" training course in Texas to learn how to fly WWII planes.
In part 7:
http://www.airbum.com/pireps/PirepMustangBurch.7.html
the author mentions the glide ratio of the Mustang.
"I was at 6,000 feet and I thought I'd never get that thing down from altitude.
At 175 mph it has a glide ratio of nearly 15:1; it will glide three miles for every thousand feet of altitude."
Another interesting comment:
"The Mustang was built to be flown by well-trained 200-hour pilots. I repeat, well trained, and if a pilot gets that same sort of training, the Mustang will be a piece of cake. On the other hand, if you approach the Mustang figuring you can whip it because you have thousands of hours in Bonanzas and the like, it'll chase you all over the airport. The P-51 has characteristics that nothing in civilian aviation can prepare you for."