Hi Darren,
Marketo will not automatically sync leads up to SFDC. It requires first a "Sync Lead to SFDC" flow action.
If you want leads to automatically sync to SFDC when they are created in Marketo, create a trigger campaign with the trigger "Lead is Created" and put "Sync Lead to SFDC" in the flow.
If you want to sync all leads that are in Marketo but not in SFDC, create a Smart Campaign with the filter "SFDC type is empty". This will pull a list of all leads who are not currently in Salesforce (since all the leads in SFDC and Marketo will have an SFDC type".
Best,
Will