No. You never know who you might be up against. This is about a physical or mental competition or challenge, a fight, etc. "The boxer is up against the stiffest opponent of his career." You never know who you might run into. This is about meeting someone by surprise. "I ran into my old friend from high school."

No they are not the same. You never know who you might be up against - this doesn`t mean you will actually meet up with the other person. For example you go for a job interview, there are other people also being interviewed for the same job but you don`t meet them. You never know who you might run into. This suggests you may actually meet up with someone somewhere. For example you could go to some fancy, expensive restaurant where well known actors or actresses eat and you never know who you might run in to.