Hi Sugarandscrubs,
soreness in the upper thoracic/lower cervical region could be from hyper extending the neck to much to look down the road. Some growing pains are to be expected when increasing the workout but a 10k jump is not huge. If that's the case your body should adapt over a few weeks.
The fit could also be a problem. Often being too low on the bars i.e having to crane ones neck to look ahead, or being too stretched out could also do it. Ironically sometimes being too compressed can also pose some issues. Generally if the fit is right, one shouldn't have to think about how to posture one's body on the bike, it should imo just happen naturally.
I'm including a picture of a race I was in a couple years ago for reference on varying positions, unfortunately it's not a profile so not the best example but you can see my teammate behind me (far left) is quite low on the bars and his neck is pretty extended. He often suffered from quite a bit of soreness in his neck and back but was in terrific shape most of the time, a good friend, and I had no problem towing him to the finish for a shot at the podium. My position is a little higher on the bars and more compact than some of my peers but that's what I find comfortable. The little fella in front of me (on the right) has his shoulders scrunched up around his neck. He doesn't look too happy either, but granted it was the bell lap in a race
Have you had someone at a good shop evaluate your position? It doesn't need to be a pro fitting and a basic one should be included along with the purchase of a new bike, but if that was some time ago a basic fit should only cost a few bucks, no more than 20-30 minutes of a bike shop employees time. Know that even if well intentioned, a fitter may still be imposing their own notions of fit into the picture so sometimes a second opinion is needed. Fitting is a two way street and quite a personal thing so a good fitter should want your feedback. If you have your saddle set up at a good height and fore/aft position, and it's not a matter of just getting used to the increase in mileage, it sounds like you may need some stem height/length tweeking. If so, best to keep changes in fit made incrementally, i.e. no more than 5-10mm at a time as a little bit can often go a long way. In the absence of a fitter, the internet has some good resources available (Youtube is a good place to start), so just doing some searches on bike fit will yield a ton of info that all seems to share a common delta. Start in that ballpark and move out from there.
Dan
