Honestly... I say go with what your are comfortable with... anything over 60 is pretty "safe" on your joints (at least, that's what my PT has always told me). If you were much under that, then I would worry.. but 65-70 is fine... especially if you really aren't moving very fast - I think a slower cadence puts more stress on your joints at 20mph than it does at 12mph.

I think keeping a steady cadence (>60) and moving at your own pace is more important than trying to hit 85 if your body just can't keep up, KWIM? I think it will come in time.. like I said yesterday, I was surprised mine was as high as it is, because I really thought I was still pedaling much slower (I know I was slower starting out). I definitely notice that I am moving faster in the same gears compared to a few months ago.

As far as I am concerned you build leg strength either way... I think it's the amount of time you're actually out on the bike more than the cadence you maintain.