You'll get used to it... Just need to strengthen the abs and lower back...
One thing that will help, is do some stretching before riding, just some 5-10 count straight legged toe touches will loosen your hamstrings, hips and lower back..
I actually have to do those all the time from a sciatic nerve injury I had doing heavy dead lifts when was 19. The physical therapist said those toe touches really help not just your hamstrings, but hips and lower back as well-which is what I really needed with that pinched nerve in my lower back running down through my hips and ball joints in my leg/hip area. They really will help you work the kinks out.
btw, I rode a Harley Sportster as a trade with a riding friend one Sat afternoon for about 200 miles. I was used to the Bandit, and doing 300 was easy for me on the B12. Man, within 100 miles on the Harley my ass was asleep, legs going numb, and that night my tail bone felt sore. I had to take a walk to wake everything backup below my lower lumbar.
You just need to get used to the new position is all, it is actually better for yor body in the long term-establish good posture so everything is supported ergonomically. Bad posture does eventually lead to injury prone stuff, you're just used to the bad slouch cruiser position, you probably had to go through growing pains with that bike too when you first started.
Try the toe touch stretches, they do help loosen ya up before, and pull the stiffness out afterwards.