Getting in shape but heavy legs

Question
I have been starting to increase my miles each week, do a track workout, fartlek with some long runs in there.  I am starting to feel my lungs are fine and I dont really get out of breath but my legs are nowhere near where my lungs and heart are right now.  For example, I ran an 8 miler today, longest run I have done in a long time.  I ran a steady easy 7:00 minute pace and I wasnt out of breath at all at the end but my legs were really heavy and exhausted by mile 5.  Are there any workouts or things I can do to bring my legs up to my lungs?  Thanks.

Answer
Hi Cody
Yes,very your workouts with fartlek with various changes of pace such a 2 min fast 1 min easy...5 min fast 3 min easy.You pick the rest recovery numbers at any distance time and distance.
You can also do hill repeats of a variety of distance from 100 meters to 800 meters.The body responds to overload to reach a higher level of fitness on a consistent basis based on what your trying to accomplish with your goals and end result program and desire.Any other questions just get in touch Cody.

All the best

John