Hey people, I just digged this up to say thanks for the malaria info and whatnot.
I'm now safely back and had no problems apart from the obligatory stomach infection.

I didn't get to ride a bike at all there... and there were just 2 kinds of riders in and around San Luis anyway: 1) totally broke people on ****ty MTBs and on age-old rusty coaster brake singlespeeds; and 2) the occasional roadie on a training ride.
Because of the above there is no bike rental place in SLP even tho it is a decent sized city (~800,000).