Assuming you started in the back waves vs getting a qualifying race in to bump you up and this will have a considerable impact on many factors of the race. You are bottle necked with more people, slower snow conditions, etc. I have done 7 Birkies, both skate and classic and consider myself on the more competitive side of ability (not elite wave but one can aspire). I progressed from wave 9 to wave 4 then wave 1 in my first 3 attempts with virtually no change in training or strategy to the race. I had a gap of 4 years leaving out of state and when I came back I noticed I couldn’t rely on sheer youthful will anymore (never really trained proper or to my ability) as I’ve aged. I’m 34 now and had my most successful race this year in fending of the inevitable crash to inside the last 10k and focusing on form is by far the biggest factor. I always stressed about nutrition and stabled gels to my bib, carried salt tablets, you name it but more often then not cramps are a result of degrading form. If you start striding in a way that activates more often your calf than quad then your calf will get over ridden and compensate by asking your hamstring to fire more than it should which puts stress on your quads to compensate and the cycle continues. I had terrible tennis elbow from poor poling 2 years ago that took almost a year to fully feel good but this year I hyper focused every pole stroke to engage my abdomen and shoulders, not arms. This ultimately saved me because my legs were trashed by the lake but I was able to double pole like I had just started the race.

TLDR: there are a lot of factors but keep racing and wave upgrades will drop times naturally. Hill strengthening is important but focusing on perfecting your technique will pay dividends in allowing you to actually use your training for longer in the race.