I skated well for me.
MTA (post-Birkie, 2 weeks later): My HR for the Dala went up above 160 bpm early on and stayed there for most of the race (2 hr 20 min), gradually coming down into the 150's towards the end.
I have no idea what my HR was during the Birkie (since I did not wear the HR strap), but I imagine it was 160-ish for long stretches, which is NOT GOOD for a race that takes over 4 hours.
I need to train with longer workouts and/or races next year before the Birkie -- maybe ski the full Vasaloppet, but at an easy pace (or the "Classic," which is not quite as long as the full Vasa, but it's Classic only, maybe 44 km or so? Then again, if I shift back to Skate, definitely do the full Vasa skating, but easy paced ...).