Laura, sounds like you're doing an awesome job training! I can relate to the idea of sometimes finding motivation to come by, but I also think heat has been an issue for me. It is difficult, and perhaps unwise, to go on long rides in humidex advosiroes and smog warnings (we've had a few of those in this neck of the woods).
As to my training, I was doing all right, I think. Didn't have problems doing 50k, but I also hadn't really gone out fully loaded. Shortly after our phone conversation, I decided to take advantage of a cool day, so I went for an 80k ride, with somewhat loaded paniers. It was all well and good, except that my knees started hurting, and I haven't been able to ride much since then (or at least, I was advised to let them rest for a bit). I think my problem was that my seat was too low, so if I can give everyone one piece of advice, it's to make sure that your bike fits you well! Also, I think going out fully loaded is a better/faster way of discovering fit problems than going on long rides without much of a load, so maybe try that. As important it is to test our gear, I think it's just as or more important to test our bodies and make sure we're good to go in a month.
Hope this helps, and keep up the great work Laura!
Marina