Spinal flexibility

Will my spine get more flexible? Because right now it's more than a little stiff. I can't get my left shoulder far enough toward (let alone past) my right knee to reach the floor in parivritta parsvakonasana and Philippa has me doing some preparations for backbending which are the bridge (setu bandha sarvangasana) with hands by my ears like I'm about to go into wheel and the chest movement of fish pose (matsyasana) but with my knees bent and my arms still by my ears (does anyone know what these preparatory poses are actually called?). And I can barely get my hips or chest off the floor. Why is everything so tight there? And weirdly, also in my chest? It always makes me laugh inside when Philippa presses my shoulders open to stretch my chest in shavasana, but it's come into even sharper relief in purvattasana. Actually, the more I think about it, the more I think I'm just upper-body stiff in general - my shoulders are tight in prasarita padottanasana C and I cannot grab my elbows behind my back. I suppose it's the danger of an office lifestyle. Somebody please reassure me things will improve!

In further contibution to the global economic recovery, I bought a new yoga mat to replace the old one that is currently leaving purple bits all over the studio floor and me. It arrived today and I'm sad that I have to wait until Friday to inaugurate it. On the other hand, I get to run tomorrow, according to my modified Hal Higdon plan. (Modified because I'm only planning to run 3 days a week, counting yoga as both cross-training and stretch & strengthen, and to add one 12 mile run into the plan a month before the big day - his beginner plan only goes up to 10 miles and I think I'll be happier with something a little longer as prep.)