Congratulations on your Tour.
I too did my first serious trek this summer. Nineteen days to destination. I too experienced this new sensation. As it never appeared in my short weekend trips building up to my Trek. I, at first did not like it at all (o.k scary). The first time was when going down a long grade, stood to cool/stretch. I avoided standing at speed, and checked everything. I was worried my rear rack was going to fail (now that would be interesting!). I found that by getting my weight lower in my Panniers helped considerably.
I kept altering my weight distribution each morning as I repacked for the day. Getting my front Panniers heavier (as mentioned above) I preferred by the end of my trip. My front racks are lower than stock mounting hardware by say an inch.

Hope this helps.


-Snuts-