1)This AIN'T nutrtional, lytes or hydration, though that can make it worse.
2) We recover slower at age and heat is as much an exertional stress as exercise
3) Get. a stress echo and note your max METS and HR recovery time, maybe peripheral arterial leg ankle brachial pressure index. Cheap basic eval.
4) maybe try logging your rides for speed distance cadence and temp to note your ceiling and go gradual

I am in Texas and the heat here bonks my 70 yo butt
but I try to stay within my limits and do a bit better every day
which sometimes means that I just showed up.
A man's gotta know his limits.