How heavy is your dog now? If you can find a Globe Live 2 or Live 3, that makes a pretty excellent dog bike. The max listed weight is 20kg, which is almost exactly 45 pounds, but I've seen the Globe's rack stand up to dork-abuse.
Specialized bought Globe and promptly discontinued the Live series, but you can still find them if you're patient. I just found one for my gf in her size.
Globe Live 2 has a traditional chain drive with IGH and U-brakes. I've added road bars and a few other upgrades.
The Live 3 has a Gates belt drive with IGH, hydraulic brakes, and Velo Orange offers these porteur bars, which are IMO more suited than the upright beach-cruiser bars.
My dog has done 60 mile rides with me, and we have no problems averaging 20+mph on flats. She weighs about 20 pounds, so hills can slow us down some. It's pretty sweet being her stoker.
(The bars are 52cm, and the leverage comes in handy when she suddenly spots a squirrel or another dog.)