I'm glad this topic was brought up.
I am currently a sophomore playing tennis at a Division II University in Tennessee. I was originally recruited to play baseball here, and did so my freshman year and first semester of this school year. As far as training time is concerned, all divisions are the same, so being a Division III athlete trains just as much as a Division I athlete in their respective sports. Here is a basic schedule I had with baseball:
5:45-7:00AM: conditioning
3:00-7:00PM: practice
7:30-9:00PM: weight lifting
Thats almost 7 hours a day that I put towards baseball Monday - Friday. Luckily, my grades were good my freshman year. Last semester, I made a C in differential equations and a C+ in organic chemistry (double majoring in Math/Biology). I never studied for DE, simply because I had NO time at all - after eating dinner, taking a shower, and finally sitting down, my clock would read 10:30PM. I have played baseball since I was 4 - it was and will always be my true love, however, I made the decision at Christmas break to give it up for my hopes in becoming a doctor.
Luckily, I played tennis just as long as I played baseball, and I picked it back up after 4 years off from it. I am proud of myself for getting my game back to compete at the collegiate level. As far as time constraints go, tennis takes up maybe 2 hours of my day - huge difference from 7. Currently making an A in DE this semester, and it is all because of the time I have to put into my studies.
Got off on a tangent there - but I must say, playing a collegiate sport at any level shows determination, time management, team work, and a desire to excel in areas other than academia. In some cases (and in mine), it is helping to pay for your education - so in a sense, it's a job because scholarships are awarded based on performance. You aren't guaranteed a scholarship for all four years. Don't worry so much about other EC's - your sport will be your biggest EC, and it is a great and respected one at that. As far as grades go - don't let it affect them too much. If you are having trouble, get a tutor, talk to your coach, etc. When I told my baseball coach I was quitting, my exact words were "Baseball is taking way too much of my time, my grades are dropping, and I'm going pro in something other than baseball."
Good luck!