Since I'm only a Freshman, it is a little too early for me to start thinking about getting my masters, and I don't really know many students in the upper level CS program to ask. However, if you would like more information on the concurrent program, you can try going here : http://www.cs.jhu.edu/current-undergraduate/adviseman.html
and just doing a search for concurrent and you'll find some decent info on it. You also may be interested in http://blogs.hopkins-interactive.com/academics/category/computer-science/
, the second blog is from a CS major who was involved in the concurrent program. I'm sorry I cannot be of much help personally as of now, but hopefully you can learn from those links.
As far as CS class, we cover a lot of ground. The midterms, for Intermediate Programming (one of the base classes for CS, which I'm taking now) generally consist of four sections. First, there is true/false, then multiple choice, short answer, and long answer. Here is the course website, if you scroll down to the bottom there are a few past midterms that you can take a look at.http://gaming.jhu.edu/~phf/2011/spring/cs120/
The final project the past few years (subject to change) has been to create your own computer game version of battleship. As far as the difficulty and work load, it is both a challenge and there will be long assignments, but it is most certainly doable. The CS assignments aren't easy--you won't be able to do them an hour before class and expect to get an A, but if you put a few hours a week into it you'll be fine. As far as manageability of double majoring, it really depends on the person and how well you manage your time. Some can manage the work better than others, so its really all about how well you personally do your work, although it is certainly possible.
Lastly, when it comes to prior experience, it definitely helps but is not needed. Intro to Programming with JAVA will teach you everything you need to know to build off of for your CS degree. Also, programming, although a major aspect of it, is not the only thing CS majors need to know. There are several aspects of the CS major that do not have to do with programming, so even if your not the best at it (and I assure you, I am far from the best), you can still get by.
Hope this helps! Let me know if you have any other questions.