The clock is ticking

Weekend commitments led me to switch my Sunday long run to Saturday, a fifteen miler. Normally I would begin a run like this at projected marathon pace plus 20 per cent and do the latter half at marathon pace plus 10 per cent. Today, I decided the good temperatures would be opportune to do an evaluation run to see what kind of shape I am in. The plan was, six miles easy, five miles at marathon pace plus 20 per cent, and the final four at marathon pace. The first five miles were north of 08:35 with a HR in the early to mid 130s, the next five miles averaged just south of eight minute milling with a HR in the early 140s - all good.
The final four miles at marathon pace began well; the first mile averaged 159 with a 06:36 split - little fast but it felt good. The second mile elapsed in 06:45, but it felt tougher than I would have liked with a 164 HR. Third mile in 06:47 and average HR of 166- not good, this almost felt like tempo effort. Final mile of 06:45 and HR of 159. Had I not been stuck waiting for a pedestrian light the HR would have been higher.
My legs have felt a bit heavy and tired since Wednesday, as a consequence of the first interval session of the training cycle, tearing up a floor on Thursday with a crowbar, and a not insignificant workout on Friday. However, I think I am clutching at straws to account for how tough this was; perhaps the five weeks of training between now and taper will pull things out of the fire? The clock is ticking...