Pace is time per unit of distance - minutes per kilometre or per mile - and speed is its opposite, distance per unit of time. Pace is just the inverse of speed - time per distance instead of distance per time - and both connect to distance and time the same way this site's own speed/distance/time examples do. Know any two, get the third, with real race distances built in.
Every distance road race is defined in kilometres or metres first - a half marathon is officially 21.0975km, a marathon 42.195km, dating back to the exact course laid out for the 1908 London Olympics. "13.1" and "26.2" are miles rounded to one decimal place, which is fine for casual reference but genuinely shifts your pace target if you're aiming for a specific finish time. This uses the real distance (13.1094mi / 26.2188mi) so a goal like "sub-2:00" comes out to the pace you'd actually need, not a slightly-off approximation.
Pace and speed are the same information read two different ways - a runner thinks in minutes per mile, a cyclist or treadmill display usually shows miles or km per hour. This shows both together whenever you solve for one, so you don't need to separately convert between them.
About 5:41 per kilometre, or 9:09 per mile, held for the full 42.195 km. Most runners plan slightly faster than target pace to allow for slower late kilometres.
Pace is time per unit of distance, like minutes per kilometre. Speed is distance per unit of time, like kilometres per hour. They are inverses of each other.
Each of these has its own page with the exact factor, a reference table and the working shown. The converter above handles any of them too - these exist so the specific answer is one click rather than a search.