exercise for husky

Question
I just got a 3-5 yr old spayed female husky,  very very sweet nature  very easy
to live with learning to walk on the long retractable leash quite well,  heeling
when we come to crosswalks, responding to a reasonable tug ( except when
seeing a cat... etc) . ( I have had a border collie, a great pyrenees x and two
australian shep herd x's  ( have those 2 still-  she ignores them...) We are able
to get at least 2 daily walks at a pretty brisk pace of a minimum of 2.5 miles
to 5 or 6 miles a day (she stays at a light little trot the whole way, not a walk)
That's my distance... hers is "more" since I can walk her mostly on a 25 '
retractable leash so lots of swings around back and forth)  3 or 4 walks a
week are in the country  up hill and down and it sure is good for MY health...
She always enjoys these walks on the leash, but is TOTALLY bored with our
yard  (8 foot concrete block fences  -- and also with the small enclosed pubic
dog runs... 1 or 2 acres.  So except when walking or going with me for a ride,
she is a totally indoor dog. I have let her off leash in the country 4 times now,
she would go off maybe 500 or 600 yards and would usually come to her
name... but not always.  She'd be just barely visible,  but clearly looking back
to keep track of me, and then return at  a high lope every 10 minutes just to
check in and get some water and a treat...BUT the last time  she got out of
sight over some hills and out of hearing range, at least 1/3 mile from me    
when I figured out I was not gonna find her I went back to the car  ( at least 2
miles).... she was waiting there, reclining in the shade "Where have you been
all this time?" .   There could not be a happier dog than she when off leash
like that, but seeing how many times one is told to never let them off leash,
and considering that it was a good half hour that she was out of sight on this
last time...  I am reluctant to do this again.  The area is roughly  3 x 8 miles  
mostly hilly to real hilly but little to no brush, bounded by roads which are
more or less rural, with some rural residences in one direction.    Judging by
her mellowness the exercise seems sufficient, but is it reasonable to let her
off leash in this area?  and when summer comes  (hot here)  I plan to adjust
time for a longer walk at daybreak, shorter one after dark.  Does this sound
like a decent plan?  Im retired  and in pretty decent health and shape  with the
time, and she goes with me pretty much everywhere

Answer
If she is mellow she is getting enough exercise. But I will admit I have a big concern with her being off leash. It is said trust is the #1 killer of Siberians. Siberians is very intelligent, but saying that they tend to zone out things like a car driving strait at them. I only let my Sibes off leash when they are in a well fenced area, or indoors. A Siberian is happier if they can run freely but I wouldn't risk their safety for it. She might be bored with the runs and your yard but she will be safe running loose in them. If you found her a doggy friend she probably would like them better. Sounds like you are doing great but the off leash thing. She sounds well adjusted and happy.
Any more questions please ask!
-Michelle B