dog-walking tips for the dim-witted

[badger]

I have signed on to walk a friend's dog while she is training for a new job. She is 1-2 years old spayed lab cross and has never had any formal training. My friend has taught her a few basic commands (sit, stay, etc.) which she obeys, but only with a great deal of prompting.
I have been out with them a couple of times just to watch her in action, and frankly the dog is a bit of a nightmare on the leash, constantly pulling in all directions. She gets lots of 'playtime' but never really long walks, since my friend has a bad back and can't go long distances.
Unfortunately, any broader strategies I might suggest would not be well-received, but even if we just work on a couple of things while I'm with the dog, to make our walks more pleasurable, it would be a huge help.
Although she is frequently off-leash in the park, this will obviously not be possible when I have her.