You have been given good advice so far. Since I can't see all the replies as I write, I may repeat what has already been said. This should not be construed as overlooking other posts or thinking you don't "get it". I am preparing to lead my 6th group rim-to-rim hike this coming May. I have also been in the canyon twice in the winter. I mention this realizing that there are others here that are more experienced with desert hiking in general but wanting to convey that there is some weight to what I will offer. Here are my thoughts in no particular order:

1) You are already in good enough physical condition -- you should certainly maintain it, though. Perhaps the most beneficial addition would be time on a stair climber.

2) Your level of physical conditioning is also a potential negative. Most of the people who die in the canyon were in excellent physical condition. However, being in good condition can lead some people to think that they are somehow immune from the "rules of the canyon".

3) If you have the ability to treat water, you should not need to carry more than 3 liters of water on any leg of the trail between water stops. Going down the Bright Angel gives you water access all the way down and you'd only need to carry 2 liters at any one time. (This is assuming your wise choice to move early in the day. )

4) If you want to have a more scenic experience, consider going down the S. Kaibab trail. It has a variety of views compared to the B.A. which looks the same all the way to Indian Gardens. It is also shorter. If you do that, you should carry 4 liters on that segment, though. There are special buses that go early in the morning directly from the Backcountry Information Office to the trailhead. Otherwise, the regular bus system can get you there.

5) Be on the trail by dawn (not sunrise). This means being prepared to do breakfast, camp chores and packing up with headlights. Also, check on the time of moon rise and set to know whether a moonlight hike can be an option. It can be a marvelous experience.

6) Realize that your training in Florida can only approximate the environmental conditions of the GC in the end of June --perhaps even deluding you into thinking you really know what it will be like. It is not unusual to see the thermometer pegged at 120*F at that time of year. And the dryness is overwhelming to anyone not adapted to it. Additionally, because of the altitude, the air is thinner, which not only dessicates your lungs and provides less oxygen (at the level of the Colorado river, your home in Florida would be about 2400' underground) but it also makes the sun "hotter" than the air temp might indicate. Your status in regard to dehydration will fall into only two categories: 1) You are rapidly becoming dehydrated and must take preventative action, or 2) You already are significantly dehydrated. This is serious because dehydration can cause serious physical and mental problems which will not be prevented or reduced by any of your physical training. Your ability to cope with the constant dehydration is the primary factor determining the success or failure of your trip.

7) One use of water is for soaking. If you will be hiking in the sun, you should plan on using this strategy as a matter of course -- not as something to hold in reserve in the event you start feeling bad. Soaking a Packtowl (or similar) and wrapping it around your neck can be an effective way of getting evaporative cooling.

8) Bring a broad-brimmed hat -- and a string or other means of fastening it to you. If the wind blows it off (not unlikely) you may not be able to retrieve it as it sails off 1000' below you.

9) Don't bother with tents in the canyon at that time of year. (On the rims, yes.) They aren't worth the weight. A simple tarp can protect you in the unlikely event that you have a rain shower. Also, do not bring overkill rainwear. A cheap plastic poncho will be just fine. (Verify weather forecast before going over the edge in case some major weather system is moving in).

10) Rest often, eat often. Take breaks early, before you feel tired or muscle stress. This will pay dividends later in your hike. Also -- particularly on your hike out -- you will need to eat. Minimally, your caloric requirement will be at least 4,000 kC. (probably much more) that day and it will not all be in your muscles or liver. You must eat or you will "bonk".

11) People are right about the hiking poles. Definitely plan on using them. Also, when I take groups, our pack weight is about 22 lbs. If you carry more than that, you either have too much stuff or not the right stuff.

When you go up to Cottonwood, I would suggest getting there early to get the most shaded site. (The nicest one is the group site, but don't set up there. You will be evicted.) Cottonwood can be an oven, even though it is over 1500' higher than the river at 4,000'. I'd set up camp in the most shaded site, make sure that the food is in the ammo cans and lids properly fastened (I had a mouse sneak in this past Jan.) and then head back down the trail to Ribbon Falls. Stay there until 5:00 at least, playing in the creek and exploring around. I like eating lunch or supper up on the ledge behind the falls. Bring food in a daypack, but do not leave the pack unguarded. The local squirrels own the spot and will be into your pack in less than 5 minutes if you walk away from it (or even turn your back and don't pay attention).

This would be my strategy for the hike out: Start early, have breakfast and head out before sunrise. Pack 2 liters of water per person. (You are still by the creek). Stop at Roaring Springs for rest, rehydration and "second breakfast". The next leg, up across the Redwall Bridge and through Supai tunnel will be your big push of the day. Carry 3 liters on this section. There will be a shady resting spot somewhere near the big pourover where the trail makes a sharp turn to the right along the section where there is the most dropoff from the edge of the trail. (Trail is 6' wide so no real sense of exposure). Exact location of shade depends on the sun location, of course. There should be running water at the Supai rest area. Toilets & running water are at Cottonwood, Roaring Springs and Supai. (Check with a ranger for any late-breaking news about water interruptions). You should carry 2 liters for the remainder of the hike. Coconino overlook is a good place to rest for 15 min. or so. In my experience, after you reach Supai, the rest should be piece of cake, comparatively.

I'm sure you must have looked at the DVD that came with your permit. I have some training and gear info that used to be on the Grand Canyon Field Institute's website but which wasn't there last time I looked. If you'd like a copy, send me a PM with your email address.

You are going to have a marvelous time in a place recently rated (once again) as the number 1 scenic location in the US.

PS In regard to monitoring dehydration, urine quantity and color is your best guide. The rule is "pee pale". It should be a pale yellow but NOT clear, which can be indicative of hyponatremia -- potentially more deadly than dehydration. Also at least a few militers every three hours or you are dehydrated.