Stock seat is awful.. When I bought my KLR I had to bring it home from Belvidere in my truck.. I then rode from my place to Freeport to buy an oil filter.. I didn't make it to Dakota before knowing that the seat would have to be changed..
I'll do this for you, though.. I'll swap you my Russell Day Long for your stock seat to use during the trip. And you're welcome to anything else you need that we can pull off my bike.
As far as on the highway, it all depends on tires.. Email me for way too much KLR info..