Wow. That is seriously slow for 50G.
Ok, there are a couple of ways at looking at this:
1) The snapshot too old message means you don't have enough undo to keep the view of the table consistent, but...
2) It's hardly surprising you don't have enough undo, as the export took 24 hours to get this far.
I think the first thing you should do is target the performance of the export. Once you start the export, you can identify the session running the export and check what it is taking all its time on. A really useful tool for that is Snapper.http://tech.e2sn.com/oracle-scripts-and ... on-snapper
Of course, if you have diagnostics and tuning pack, you can use the Top Activity screen in Cloud Control to monitor the session.
When you see what the performance bottlenecks are, you can make some decisions about how you can approach tuning the export.