When you change to a new exercise why is it so hard?

Shyoutgoing
I just went for my first real run tonight, I usually use the elliptical. Out of the 12 blocks I probably ran 4.. It was so hard! On the elliptical I can go up to high levels and spewed through it! Why is it so hard to run normally? While I was running I could feel my abs tighten (I do an exercises too) to the point where it was giving me cramps. Is that normal?
When you change to a new exercise why is it so hard?
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