Hydrate like crazy and keep up your caloric intake. Also, since you are hydrating so much, be sure to take electrolytes as needed. I trained for and ran a marathon while DS was still BFing. I had to eat, eat, eat. I got tired of eating I had to eat so much!! You'll burn about 100 cal/mile. While training I always wore a Camelbak, and sipped constantly. I absolutely could not have done my training without it. I used Endurolytes for electrolytes, and fiddled with the amount based on the length of the run, temp, and humidity. I used Shot Bloks for nutrition/calories during the run, and had a Clif Bar immediately after.
Also, don't get too freaked out by a few days of not getting much out of a pumping session. This often happened to me with that bedtime pumping when I was nursing DS, but my supply was fine. Your DD is at the age when she may be getting some of her caloric needs from solids now, so your supply will naturally decrease some.
mommy to DS who is 9
DD who is 6
and my girl in heaven