Hi! MAny of us did that transition
There isn't a single equivalent lens, although the 28-105nis your safest bet. It's compact, has a nice range, and is better than it has any right to be.
The 24-70 is much bigger and with a shorter reach, although in some cases the faster aperture is invaluable.
Based on experience hiking, I'll give you my humble opinion, to use as you see fit.
Get the 28-105 as your main driver. Supplement it with one or a few fast primes such as the FA 43mm if you need the faster aperture (while understanding that the primes are generally not WR). The extra reach of the 18-135 can be compensated by cropping your 105mm images. If you truly need a longer reach (and while hiking weight is an issue so that's something to ponder carefully) the DA 200 and 300mm are FF compatible, as is the 60-250mm if you are willing to modify its rear baffle (there's an article about this here on pentaxforums). Or the 70-210mm is a more direct option with faster AF albeit with a shorter tange than the 60-250. All of the tele lenses available for FF will be fast and somewhat big, however. They'll all be WR luckily. We still lack a modern 70-300 equivalent.
One left field option would be to get the Irix 150mm macro, it's a manual focus lens, but auto exposure, it's WR, doubles as a macro (convenient), is moderately big, and extends your reach.
I hope this helps.