In 2004, I trained to run the Boston Marathon. I live in NE Ohio. Most of my training runs were done in at or below freezing temperatures. I was prepared to run a 3:15-3:20 marathon. I was running 70 miles a week in preparation. I was good to go.
Fast forward to race day. April 19th, 2004. The start of the race in Hopkington at noon... it was 86 degrees! Weather fluke. I've never seen more people hauled off a race course anywhere. Out of the 18,000 people who started, over 1500 did not finish. I hobbled to a 4:32, my slowest marathon ever.
I suspect that my difficulty was due in part to the heat of that particular day. If it would have been 50 and overcast, things would have been entirely different.
You should train as you will expect to race. Typical April Boston weather is not 86 degrees, but obviously it had something to do with my poor performance.
If you are running for fitness only, as long as you make sure you are properly hydrated, run when you can, whatever time of day.