Getting warmer - getting slower?

Cop-out

Up until a few weeks ago I have been steadily improving. I'm knocking out 50 miles a week and follow a reasonable routine of easy runs, tempo, interval and LSRs. As an indicator of my improvement I usually do a 6 miler once a week maintaining 75% MHR along the same route. In 7 months I have bought this down to 8:16 m/m. Why then am I suddenly pulling 8:50's or worse? Is it heat or am I getting old and crap?

[Deleted User]

Doesn't the heat have an affect on your heart rate anyway? Would have thought that the heat would have meant an increase in your effort meaning that 75% of your maximum heart rate has gone up when it's warm.