I don't know if you have anyone driving and helping you, but you could do the first two days without any load: HI (Happy Isles) to TM (Tuolumne Meadows) and TM to RM (Reds Meadows).
The first leg is not bad (I've day hiked it, only the opposite direction). The second is a long day. I think AlanK and his son have done that one in a day. (Hoping Alan will describe his experience.)
My son and I did the JMT in late July, 2006. We did indeed do HI to TM the first day and TM to a bit before Red's Meadow the second (we took a bus into Mammoth to pick up supplies and permits -- that was our only supply stop). The first day was about 8.5 hours on the trail -- a bit short because we wanted to stop at TM. The second was our longest -- about 12 hours on the trail. But we were hiking, not running (although Eric would probably have agreed to run at any moment). Our overall time was 9 days, not 5. If you want to average 40+ miles per day instead of 25, you won't want to stop at TM for a night.
This year, even in August, your only water (quantity) issue should be in the Whitney Zone, where you can expect to go from about Guitar Lake to trail Camp without a refill. That's about a 3 quart jaunt. Check Wenk for this, but I would be surprised if you need to carry more than a quart anywhere else. Water quality is another thing to look carefully at. More and more people are dipping and sipping, some still using filters, UV and/ or iodine.
I agree. We each carried 2 half-liter bottles the whole time. We filled them directly from streams without UV or filtering. We had platypus bags for extra water at camp each evening. We also used them to take an extra 1.5 liters each up Whitney, from somewhere before Guitar Lake.
