Wipe hard drive and delete recovery partion.

1. Shut down.
2. Disconnect all external storage, except for the bootable installer. This as a data-preservation-safety step, and also to reduce how much storage info gets displayed.
3. Boot from the USB installer. Steps used here listed in the installer-creation support article.
4. You’ll eventually get the first screen of the macOS installer displayed. The OS X El Capitan installer, if that’s what you’ve used here.
5. From that first screen of the installer, either select the Utilities entry from the menu, or select Disk Utility directly if that’s shown. Details on that first screen of the installer varies by macOS release, and i don’t recall what El Capitan showed there.
6. In Disk Utility, select the whole device in the left part of the Disk Utility display.
7. That’ll usually be by selecting the vendor and model name of the storage device.
8. That is, select the entire storage device, and not a partition co-resident on that device.
9. Erase, and (re)format as GPT. Erasing and re-formatting—erasing is for hard disks, and unneeded on SSD—will clobber the recovery partition.

Download El Capitan, then build a USB bootable installer, boot that USB device using the steps in that build-an-installer link, select and wipe the whole disk using Disk Utility erasing and creating a GPT HFS+ volume, then install El Capitan.

If you plan to keep this Mac for a few more years, replace the hard disk drive with an SSD. That both for reliability—decade-old hard disks are increasingly failure prone—and for speed. OWC is a vendor that specializes in Mac storage upgrades.