I'm looking for thoughts and advice on buying a new bike...
My specs: I'm heavy, but working on losing weight. I've also got big (heavy) breasts. I'm 5'5, and currently about 250lbs, but relatively active and fit. I exercise regularly (5-6 days/wk 60-90 min) and enjoy swimming, spinning, walking/hiking.
My current cycling experience: I ride an Electra cruiser around town. I also have a Bike-E recumbent that I ride with my SO for fun on bike paths. I spin 2-3 times per week, but have never done any serious or consistent road cycling.
How I plan to ride: I would like to train for, and ride in the AIDS/LifeCycle San Francisco to LA ride next June, which is 545mi in 7 days. This means that between now and next June, I'll be building up to longer and longer distances, with rides of 70 to 100 miles in the final month or two. After the event, I hope to continue to ride often, though probably distances under 50mi.
My concerns: Being as big as I am, I'm putting a lot of pressure on some fairly tender bits. While I intend to lose weight as part of my training process, realistically I expect to still be around 200lbs at the time of the event, and have some concerns that extended time in the saddle could be damaging. I'm confident in being able to build my leg and cardio strength, but I'm worried that I'll be too uncomfortable to make the longer rides, especially in the forward posture of a road bike. On the other hand, I've had several people advise me against a hybrid with flat bars for the kind of distance I'll be doing month or two before and during the event.

I've test ridden about 6 bikes, 2 more traditional "road" bikes, 4 "hybrids", most of which were women specific geometries. I need to make my mind up and buy a bike already, but I just don't feel confident about any of it. I'm looking to spend under $1K, and so far the Trek 7.5 fx WSD is the bike I've liked best.

I deeply appreciate any wisdom, advice, suggestions, experiences, and perspectives you care to share. TIA!
Teppi