2 and a half hours on the road??
One word : displacement! The more the better.
If your talking new bikes, i'd say XR650R -w- dualsport (enduro as us old farts would say) kit.
If we're talking used, maybe an older XL-600R, XT-600??
(still has the power, but with lower seat height)
I have an '82 XL500R -w- MT21's and stiffened suspention, and i love it!
Chugs up hills and tight trails that give sub 400cc machines fits, and will cruise for hours at 65+mph.
(yea, it's heavy as hades, but that just makes you stronger!) :p
Mt-21's work great but they seem to wear out quick.
(about 1,500-2,000 miles rear , quadruple that for the front)
Smooth on the street, hook up pretty good in the dirt!