First, well done on 600 miles in 7 weeks, thats good going for someone just starting out. Careful not to overdo it too soon though, I've seen a lot of people get nice expensive bikes, ride every day for a month and then never see them again, nice bike goes on ebay about 2 years later having sat in the garage since that first month.

Definately practice changing your tube, its not that hard but you'll be glad you practiced when it happens on the road, getting the tyre bead out and back in can be a frustrating process the first time until you know whats what.

Also v70cat is very right, proper bike maintainance is essential, it will reduce the chance of things going wrong on rides, and will add years to the life of you bike.