marketo training mode

No.  All of the data and assets are live and most are saved automatically whenever a change is made. 

You may be able to set up a training environment using Workspaces and Lead Partitions (What are workspaces and lead partitions?) - we haven't done this yet.

We created a Test Program in which we can experiment with various assets, but remember if you run a smart Campaign, live data will be affected.

You may want to vote for the Marketo Sandbox Idea.

One thing you could do is create a hand-full of test leads, say 10 to 20 of them. Add them to a list called "Test", then, when trying out new campaigns make sure you have a filter of "Member of List Test". That way, whatever happens, will only impact the test set of leads.

In support we will do this all the time when troubleshooting campaigns. Add ourselves as a lead, set a filter of "Email address is (my email address)" and run through the campaign.