Unfortunately, the dry nose theory is just an olde wives tale/myth.
Watch your dog. If he/she seems to be having trouble keeping up & is laggin behind, he/she's had enough. I'm in Louisiana so I can sympathize with you on our crappy 100degree weather with 90% humidity. It your case it's best if you split up the exercise regime into 2 or more sessions (take the total amount of exercise time/activity & divide into 2 & there you have the duration for each session). This is not only because of the weather, but because your dog is still growing & too much strenuous exercise can harm joints, tendons, & ligaments (& even muscles) in a growing dog.