How close are you to your wireless router? Maybe that would help if you simply position your Macbook closer for a better signal.
Be sure to restart your Macbook, then try the download without launching any other apps.
Do it in early morning when there is less internet traffic. This can be helpful if you have cable internet.
Another way to get El Capitan that does NOT use your internet connection:
Go to your local Apple Store, and ask for a copy. They can often put the System of your choice on a flash drive. I've not done that myself, but have two friends that do that occasionally.
Or, the Apple Store should have a decent wireless network, take your Macbook to the store. You can download what you need through the store's connection. You could expect that to take less than 30 minutes, maybe a lot less.