Saturday run

Had to work today do I didn't get in my swim. Ran 10 miles 1:35:14 ave HR 150, big negative split, 49:18 out, 45:56 back. I felt flat going out then around mile 4 I started to feel much better, very strong after I warmed up, I let the HR go up to 155 so I could get in some tempo work, I know, 10 miles run and doing tempo work? Well I know I'm not going to gain much more fitness in the next few weeks so I feel that I can get more efficient by letting myself run faster, I'm just at the very top of zone 2 for me at 155. Time will tell if this theory of mine works, it feels very comfortable.