I also charge $60/hour including round-trip travel. This is significantly more than my hourly shop rate and reflects the fact that I don't like to hang pictures. I don't do scaffolding.
If you do this, make yourself a tool kit with everything you could possibly need so you don't have to make more than one trip. Mine has tools, a large assortment of hangers, glass cleaner and rags and band-aides. Resist the temptation to "borrow" tools from this kit for other purposes. It's embarassing to have to ask for a hammer, especially if you're charging $60/hour. Also, you'd be surprised how many homeowners don't have a hammer, or have no idea where it might be.
Also, find out ahead of time if you're going to need a hammer drill and masonry bits, special security hangers or pepper spray for the family dog. (I love dogs, but not when they've got ahold of my cuff and are trying to pull me off a ladder.) Assume nothing, especially if you're hanging pictures you didn't frame. You might get there and find this picture has a single sawtooth hanger. Sometimes it helps to have an extra pair of hands. A teenage son or daughter works very well, especially if they're looking for a little movie money. (If you don't have a teenage son or daughter, I don't recommend acquiring one just for this purpose.)
The best thing about hanging pictures is that I've rarely left a site without bringing back several new projects to the shop. The prevailing attitude seems to be, "As long as you're here . . ."
As an alternate plan, I've developed a good relationship with an ex-framer who also has good interior design skills. I'll often refer hanging jobs to her and she, in turn, sends/brings framing to me. Everybody wins.