I teach in a grad school and I have no problem with the idea that your real interest while in grad school is to focus on your running while supporting yourself and getting a grad degree. But you're asking a question that's hard to answer as different schools and departments are different.
The real "weeding out" in the academic world is done early, usually in the freshman and sophomre years. Once you're in a grad program they really want you to finish. I found grad school much easier and less work than undergrad and I've known many people who have had similar experiences. However, there are disciplines where the general idea is to "bust chops". Medicine, law, and social work (believe it or not) come to mind. Also, in many cases a school that has just started up a program will really pile the work on in that program in the belief that people who rate the program will equate hard work on the part of the students with a high quality program, whereas an "established" program may be more easy going. So I think the answer here is that you're going to need to do some looking around and research. State colleges often have graduate students with a "variety", shall we say, of academic qualifications and if you're on the high end of that scale you'll likely have a fairly easy time. But such schools have less money to give out. They all have some, just less.
If you're serious about this you might want to look at
. That'll give you a starting point. They list majors at loads of grad schools and have links to places that claim to be able to help you finance the program. (I have no idea if the links are really useful). You might also want to talk with someone at your own school's department of whatever grad major you're interested in. Or just stroll through the halls in the building that the department of whatever grad major you want is housed in. Every college I've seen has advertisements from other university's grad programs on the wall. Once you've identified a place you want to go, it would be good to meet some people who are in the program there to get a sense of what it's really like, but even then you'll likely get conflicting information. You also need to look at your undergraduate career and see what courses you did well in with a minimal effort. If you really struggled with courses where you had to write a lot of papers, then a grad program in English is going to be hard for you regardless of the school. It also wouldn't hurt to make an appointment with your school's career services office.
It's unlikely that an internet message board is going to give you the information you want, though there's always a chance and it can't hurt to ask.