You don't have to be an exercise physiologists to know you cannot run as fast or as long in the summer.
One of the reasons long training runs don't work out as well is glycogen depletion occurring sooner in hot weather. .This seems to be a fairly well demonstrated physiological fact.See here. Of course, volume depletion is a more dominant limiting factor.
First, a brief taste of stylized "glycogenology". The classical 70 kilogram person of physiology textbook lore carries around about 100 grams of glycogen in the liver and about 500 grams in muscles.Liver glycogen can be broken down and released into the blood as glucose while muscle glycogen can only be directly used locally to fuel muscle action,getting ATP to the myosin heads.
After a 24 hour fast some 50-60% of liver glycogen is depleted to supply glucose for resting metabolic activities. Indirectly, muscle glycogen can function as a blood sugar source by producing lactate which can be transported to the liver and converted back to glucose (Cori Cycle).Glycogen depletion is a major factor in endurance exercise adventures and this can be mitigated a bit by glycogen loading,ingesting carbohydrates during the event,repleting liver glycogen before the event and by lots of training which hopefully shifts the fuel mix somewhat to fat utilization delaying the time of glycogen depletion.When that occurs you slow down appreciably as muscles fuled mainly by free fatty acids cannot contract as rapidly.
So, maybe if you can keep cooler you can delay glycogen depletion.
One thing you can do to keep cooler seems to be to drink ice slurries.
I quote from an article in the Scandinavian Journal of Medicine and Science in Sports By authors Tan and Lee from the National University of Singapore.See here for abstract.
"The ingestion of ice slurry during exercise is a practical and an effective strategy that greatest the greatest heat sink because of the additional energy required to effect a phase change from solid ice to liquid water.A smaller volume of ice slurrry ( as compared with that of cold drinks is required to achieve similar reductions in body core temperature and improvements in endurance performance."
An earlier paper by J Dugas compared running times in the heat ingesting slurries with cold water and found his subjects could run further before exhaustion with the slurry. See here.
A similar study from Australia by Siegel and co authors also showed a increase in running duration ( about 20%) in the heat when cold water ingestion was compared with ingestion of ice slurry.See here.
The ice slurry function as a Heat Sink, a concept well known to folks who fiddle around inside computers.The small ice particles have a high surface area to volume ratio which facilitates heat transfer.
If you like snow cones you might give it a try on a hot summer days. I find the slurries refreshing and fun to eat whether my endurance is enhanced on not.