For a cross country runner, as for any other physical body, the instantaneous speed may vary over time. He may slow sometimes and accelerate at other times.
It is difficult to measure the instantaneous speed over the total distance. Moreover, although this hypothetical data has all the information about the movement, it does not answer the question of whether the runner was fast overall for the distance or not.
For such a situation, the notion of the average speed was invented. It is simply the total distance divided by the total time spent. It is the same as the real (instantaneous) speed if the real speed is constant throughout the run.
In our case, the average speed is `(50 km) / (25 min) = 2 (km)/(min).` But `(km)/min` isn't a generally accepted unit.
The SI unit used is `m/s.` To convert to this, recall that 1 km = 1000 m and 1 min = 60 s. The converted value is `2*1000/60 approx 33(m/s).` You could also convert it to km/h: `2*1*60=120((km)/h).`
This is an impossible speed for a runner. More likely, the runner went a distance of 5 km, not 50.