The SI unit is meters per second. T 4 s 1 s 5 seconds. The unit for velocity is meters per second ms.
Now that you have the individual sprint velocities you can average them together.
You should use the average velocity formula if you can divide your route into few segments. This one requires basic calculus. To calculate velocity we calculate the displacement from the starting point to the finish point and divide that by the time covered. The delta symbol D just means change in so DsDt means change in position over change in time.