Scott,

50VDC at 5A would yield a power of 50V x 5A = 250W. Roughly approximated winding of motor would present 10Ohm resistance.
Maximum heat dissipation of darlington would occur at 25V motor voltage and 25V across Darlington. 25V across motor would cause to flow 25V / 10Ohm = 2.5A through motor and Darlington. So, heat dissipation of Darlington would be up to 25V x 2.5V = 62.5W. This is quite a lot!

Using PWM, like Michael stated, would highly decrease heat dissipation of Darlington or else transistor used for this task!! Either transistor is fully switched-off, then heat dissipation is zero, or transistor is fully switched-on, then only very low heat dissipation caused by rather small saturation voltage of transistor must be taken into account.

Kai