Okay... Once I tell you how to do this, about 37 people are going to jump on and tell you not to edit the twentyeleven theme, and that you should create a child theme for your modifications.
And they would all be correct! :-)
If you modify the twentyeleven theme, you will loose all your changes when the theme is overwritten on your next WordPress update.
That being said, I believe you can change the blog page to show just excerpts by opening content.php and on on line 41, change only this;
...and save the changes. I think that might let you show your articles on the blog page as excerpts. Make sure you make backups before and after the changes. just in case something doesn't work out.
Also, look into this when you have some time: Child Themes.
Using a child theme will allow you to take advantage of any updates to the parent theme, while not losing any of your customizations during an upgrade.