Congrats - I completed my first STP on Saturday as well! Actually it sounds like we had very similar days: Same speed, same saddle time. Similar FTP as well. Unfortunately my ride partner had to drop out a few weeks ago, so I was freelancing it, forming groups where I could and riding alone for some significant chunks. I also made it to Spanaway for my first stop and then Centralia College for the next, with more frequent stops in the second half, including a longer stop where my wife met me in Castle Rock. Fueling for me was good - 6x 90g carb bottles plus food at the stops. I also proactively took some salt capsules a few times just to augment and a caffeine tab at a mini stop around mile 165. No cramping or bonking and hydration felt good. Saddle comfort and position was good throughout, with a bit of hand soreness at the end being the most notable issue.
I haven’t been following a specific TR program - kind of self-coaching using the TR platform. Obviously the STP was my longest ride YTD, but I did a century a couple months ago and a climby 86-miler last month. Definitely lived the TR mantra that you don’t need to do a 10hr ride to be fit enough for a 10hr ride. lol