I wonder what reasons the guide gave for not wearing jeans. I don't wear them when hiking for several reasons because they are usually tighter than many other pants and this can lead to chaffing on long walks, especially so if they get damp or wet. Then, as others have mentioned, if they get waterlogged they take seemingly forever to dry out.
My usual remedy is to wear army BD cargo pants, they're roomy and comfortable (one's less prone to chaffing), they've big deep pockets so things don't get lost as easily and they're rugged and take a lot of rough treatment. Moreover, they're only a fraction of the cost of spoofy/fashionable hiking attire (or at least the ones I use are).
That said, these army BD pants are cotton and like jeans will hold water although for some reason they dry much quicker than jeans even though the fabric weight is similar. To overcome the water/dampness problem I also carry a spare pair of pants made of lightweight synthetic quick-drying fabric. These I roll up cylinder-like and put in a watertight plastic bag at the bottom on my pack (the fabric is so light and flexible they takes up very little space). I also do the same with a spare shirt/T-shirt.
There are many tips I could give those who are thinking of taking up hiking but I'll limit it to one—for safety's sake don't hike alone.