Obviously, (sentence)

This means "I know this is true, and I'm sure you know it too." Use it when you say something that the people listening already know.

Obviously, changing careers is not easy, but it is possible.

not exactly (something)

This is an example of understatement. If you're talking about someone and say:

She's not exactly poor.

...it means that she actually has a lot of money. Instead of simply saying "She's rich", you might use this expression in order to sound witty and intelligent.

Here's another example: if your husband is overweight, but made fun of you for being a little heavy, you can say:

You're not exactly in the best shape yourself, you know.

"Not exactly" can be followed either by an adjective, or by a noun:

A: You're dumb.

B: You're not exactly a rocket scientist yourself.

(someone) is at peak physical condition

A person who is "at peak physical condition" is as fit and healthy as it's possible to be. You can use this phrase to describe athletes competing in the Olympics or who are very competitive in a professional sports league.

This might be the time for Dante to go for the title. He's is at peak physical condition right now.

You can also say that someone is not very athletic by saying that they are not at peak physical condition.

who (is someone) to talk?

Some people have authority to criticize something because of their experiences. For example, if you work really hard every day, it's OK for you to criticize someone who doesn't work hard.

When someone doesn't have authority to criticize something, you can ask "Who are you to talk?" So, for example, if your messy roommate complains that you don't clean enough, you can say:

Who are you to talk? You're just as messy as me!

People also use this phrase to talk about themselves, to admit when they've said something that they don't have authority.

He's so disorganized. But who am I to talk? I can't even keep track of what day of the week it is.