Except as permitted under Proposition 215 and SB 420 laws, persons age 21 and older may:
use marijuana in:
a non-public place such as a residence or
a public establishment licensed for on site marijuana consumption.
There are already anti-smoking laws in place that prohibit you from smoking in many (all?) public places (at least in SF). Worrying about your freedom to breath clean air isn't in any way related to prop-19, it's about smoking in general which for the most part has already been dealt with.
In SF you are allowed to smoke cigarettes outdoors most places except near public doorways. You are definitely allowed to smoke while you "walk around town"
15 years ago you could smoke in bars and restaurants too. Next year may see restrictions on smoking in beaches and parks, although I have my doubts about how effective that will be.
If you drink alcohol on the street other than at a licensed outdoor event, a police officer can make you throw it away or issue a citation resulting in a fine. This seems pretty effective overall.
Smoking is officially illegal almost everywhere in San Francisco. The ban was extended this year to include most places previously exempt. If you are smoking anywhere besides on a curb in front of a parking garage, you're probably violating the law.