Hello,
I'm looking for advice on whether it's normal for my back to feel tired and stiff on longer rides?

I commute 4-5 miles each way every day on a Surly Cross Check with no problems. I am pretty sure that i am riding the correct frame size, and the bars are Velo Orange flat bars with a slight backward sweep. They are slightly higher than my saddle. Bike feels great on short rides.

On longer rides (30-35 miles), my hands get sore, and I wish I had more hand positions available to me. I have toyed with the idea of swapping out the bars for drop bars so I can have more options, but my back is already feeling stiff and achy after longer rides, and I am afraid the more forward position will make this worse.

Is this something that will get better as my torso gets stronger? Do I need to work on flexibility? Is it a fit issue?

Thanks!
Gillian