Exactly. And heat acclimation is real training in its own right. Two to three weeks of consistent running in it and you will claw back a chunk of those beats, though never all of them.
Heat, and 12 beats is a completely ordinary amount for a jump from 9C to 27C with humidity on top. Your body is sending a large share of blood to the skin to shed heat, which means less returning to the heart per beat, which means more beats to move the same oxygen. Humidity makes it worse because sweat that does not evaporate does not cool you. Run by effort for the summer and accept slower paces at the same heart rate, and you will find that in September the same effort produces paces you have never seen before.
Try a drink mix instead and skip solids entirely. Getting the carbohydrate in dilute form across the whole run rather than in two spikes suits some stomachs far better, mine included.
Worth adding that caffeine and sugar alcohols in some products also cause this. Read the label for sorbitol or maltitol, which are surprisingly common in chews.
Mostly a feedback problem rather than a discipline problem. Watching pace on the watch means you correct after you have already drifted, and by then the effort feels normal so you defend it. Two things fixed it for me. First, run the whole easy run without looking at the watch at all, and instead hold a rule: you must be able to say a full sentence out loud without a breath break. Second, accept that easy running feels stupidly slow for about three weeks and then stops feeling slow because your aerobic base actually improves. The people who look effortless at 6:20 got there by being willing to look silly first.
One last thing: heavy legs on quality days is the actual symptom to fix. Easy pace is not a virtue test, it is what pays for the hard sessions.
Short sentences is the definition of too fast. The bar is boring, uninterrupted, slightly rambling speech. If you find yourself pausing to breathe mid-sentence you are running moderate, which is the pace that gets you tired without getting you fitter.
I always assumed fuel mattered for the last third of a marathon, not for a training run of 20km.
About 55km a week, so the long run is roughly 40 percent of it. That does sound lopsided now I write it down.
Agreed on rotation, but I would make the second pair another daily trainer rather than a plated race shoe. A plated shoe used for two workouts and a race is 40km of use out of a four month budget.
Whatever you buy, buy it now rather than at week 14. Running a build on a dead pair to save money for race shoes is the classic false economy in this sport.