Hi Joel,
There isn't an automatic throttle in Marketo. You would need to have separate lists of 5k each and use choices in your flow steps to send to each list separated by wait steps, or you can use separate campaigns like Mike suggests.
You can also use the Random Sample feature instead of separate lists. This would work by adding a choice to your Send Email flow step and then saying "If Random Sample is (some percentage of your total leads for the campaign(ex. 50 if you have 10k leads)) > send email (email name), otherwise "do nothing".
You'd then use a wait step and a Remove From Flow step with a choice that says "If was sent email > Remove from flow". This will remove all leads that have already been sent the email, and you can continue the flow with another Send Email flow step that has another choice and random sample combination. Does that make sense?
John