No wonder you feel burned out and want to quit.
I have a lot to say on the subject but I am running short on time.
I sweat more than anyone I know, even when it isn't hot out. I live in the south and the dew point was 78 this morning. I have learned a thing or two about heat the hard way. 70 degree dew points sound wonderful right now. Typically I run/race well during the spring, feel like quitting running completely every single summer, then run my best times of the year in the fall.
This summer started out like so many other summers but I have really committed and gotten serious about managing the heat. The result has been that I've had enjoyed my running and felt far less spent the past few weeks.
Here's what I do:
-Don't start any run later than 6am if you run in the morning. 5:30am is better.
-Don't start any run in the afternoon until 30 min before sundown when running in the afternoon.
-Stop every 10-15 min to drink water (anywhere, any way) if "hot" out. Stopping on a run will not hurt you.
-Stop measuring courses and timing runs! Leave your watch at home! Your body doesn't know pace, it knows effort. Stop trying to teach it pace!!!
-Stay out of the sun when running. Change routes, run way off in the grass on the shoulder of the road, run on trails, whatever it takes
-Nuuns are your friend. Google if necessary
-Wear sunglasses, sweat band, carry a towel with you - anything to get the sweat off. Put your head under a fountain during a water stop.
-Doubles are your friend if you're training for races lasting less than 90 min (you say you're training for the marathon.)
-Sleep well at night (this is a big one)
One question:
Not to be too critical but you do not need Vo2 workouts for the marathon. Not sure you really even need these workouts if you are racing 30 min races. The worst thing you can do is do a timed Vo2 workout in the summer (when you don't need them at all - much less for a marathon and this time of year).
I know you don't know me and you probably won't take my advice, but I know you.....because I used to be just like you.