Hard staying in the aerobic zone

I ran an easy 30 minutes on my treadmill last night. And it was tough! I had my heart rate monitor on, and wanted to stay below 150 bpm. I started out slow, and increased the speed in small increments for a warm up. But as I got up to maybe 5.5 mph, my heart rate was already starting to pop above 150. I began slowing the treadmill. I took short walk breaks. Eventually I had the treadmill down to 4.3mph just to keep my heart below 150. That is not much faster than a brisk walk. The walk breaks I took doing 4mph, and my heart rate would drop down into the 130’s.
I have few theories. One is that I am still not recovered from running the Las Vegas Marathon on 12/10. Another is that with all the training I did for the marathon, some of it bordering on anaerobic, my aerobic fitness went to hell. And another is that following the marathon, my running has been sporadic as I am recovering, and this has affected my aerobic speed. And another is the cold I am currently fighting. Chances are, it is a combination of all of these. I know before the marathon, I was able to go much faster, for longer periods of time while keeping my heart rate down. It will come back!