There are many variables when it comes to training for a marathon. There’s training volume, intensity, rest, recovery, sleep, family life, nutrition, alcohol, hydration and so on. Sometimes I can exercise control over some of these and sometimes I can’t.
The one variable that has the most visible effect on my training is ambient temperature. To plain speakers that would be hot weather.
Take my last 3 runs. All run at an average HR of 149. One was 18 miles and was run on the hottest day of the year (23 deg C). The pace was 8:16.
The next one was a hot day but not as hot as the long run. It was 6.45 miles at 7:52 pace.
Today was the same 6.45 miles on a much cooler day. The pace was 7:39.
So, if I have a shit marathon in 10 days I’ll be sending you back to this post and telling you it was a hot day.