This hot/humid weather does a job on me so don't feel bad about less than stellar results. I have a heart monitor and can easily see a rise in heart function when temps rise. Simple fact is I work harder to keep the same pace as on cooler days and I feel sluggish while in motion.
The key to a good training session on those days is to stay positive and slow down a little if needed to go the distance. It's the "harder than should be days" that can yield big gains as long as you stay within your limits making sure not to cause an injury.
D2012-adding the mile to each long day, I like for base. Don't stress yourself by going too fast, just increase the distance. Working on speed increase on the 2 mile days keeps things short and sweet so over-exertion doesn't create an injury. Focus on form/technique to smooth out your stride. Again, base miles will add strength and stamina while speed work with act on fast twitch muscles. In 4 to 6 weeks with the 10 miles a breeze, add extra distance on the short days making sure the extra is done at a faster pace.
My mantra has always been
form-function-form-function while walking to achieve efficient muscle exertion using less energy while going faster. Just because my way has worked for me does not mean it is the best way for others. I always try to have fun even though my body is telling me otherwise. I'm weird that way.
Many training plans on the net to find one that works best for personal situations.
Good luck to all.
A PERSONAL RECORD! 
I needed that!
