Yes, that "7.1-channel" capability is the MAX the on-board system can do. The 5.1 system you want to use also is possible - it's simply a matter of setting the options using the tool provided. When you are completing the install of your mobo, one late step is running the utilities on the CD that comes with the mobo. A major part of that work is installing all the device drivers needed for your system, and that includes the drivers for the audio system on the mobo. It also includes the configuration and testing tool for that system, which normally places an icon for access to it on your desktop at bottom right - see your manual, p. A-2. When you plug in your speaker system, it will show you in its menus that a 5.1 (aka 6-speaker) system for output is available to choose, so choose it. To plug those speakers in, see the diagram for a 6-speaker system on p. A-5. Your speaker system will have three plugs, each with 3 contacts - tip, ring and sleeve. One (usually color-coded green) will be for the front left / right speakers, one (usually orange) will be for the front center and sub-woofer, and the last (usually gray) will be for the rear left / right speakers.
Look closely at the manual, p. A-3, about the auto-pop-up dialog. This is part of a feature with Realtek systems to allow you to re-assign jacks for non-standard purposes. BUT in my experience you MUST respond to it properly. The sequence is that you plug in ONE speaker plug and the pop-up appears, suggesting the default device may have been plugged into that jack. If you ignore this it will disappear after a wait, but it may NOT use that default setting - it may simply NOT send out a signal on that jack! So RESPOND, even if only by clicking on the "OK" button to confirm the choice shown on the screen. Then proceed to the next speaker plug. Etc.
Once the speakers are plugged in and confirmed, you can use the configuration utility (p. A-2) to choose your speaker system and then set its options for sounds. There probably are some places where you can run a little demo to confirm the system is doing what you want, and to set up balances between speakers.
Quite separately from the rear speaker jacks, the mobo has a header for connecting to the case front headphone / microphone jacks. In the Realtek configuration utility those jacks are configured separately. THAT is where you might have an option for simulated surround sound for headphones that could be used for the front jack only, and not affect what your 5.1 speakers get.