A simple pulse stretcher built with two sections of an op amp uses voltage follower U1A to drive Dl and C2.
C2 charges to the peak value of pulse voltage. R3 and R4 determine the discharge time of C2 and therefore the pulse stretching. U1B acts as a voltage follower. Typically this circuit can stretch a pulse by a factor of 50. C2 can be charged to accommodate different pulse rates.