とは限らない is a fixed expression meaning "not necessarily", and if I were you I would memorize it as such. I'm not sure there's an intuitive way to understand it logically. For example, there's no とは限る.
As for your bonus question, your two sentences mean pretty much the same. 必ずしも is an expression which needs to go with some negative expression after it, and also means "not necessarily". It is different from 必ず, and you cannot use it to mean "necessarily" or "definitely".
Here 必ずしも...とは限らない uses both of the above expressions for extra emphasis.
必ずしも is a bit confusing in the sense that other grammatical elements can come between it and the negative that needs to follow. The below sentences pretty much mean the same:
必ずしも長生きしない
必ずしも長生きするとは限らない
必ずしも長生きするわけではない
必ずしも長生きするとは言えない
For comparison, your sentence "Living a long time is not just limited to people who exercise a lot" could be translated as:
長生きは運動をよくする人に限られない