How bad is it to run an ancient Mac Pro with an unsupported OSX?
If you have the 2007 Pro 1.1, 10.7.5 is the end of the line for you. But depending what you need to do, it's not all bad. I have a 2010 pro and run only Snow Leo, 10.6.8 most of the time. 
The only thing you can do is monitor what apps no longer function, and save up for an upgrade. $600 or so to jump to 2010 pro.