I am new to DB and have changed from fretless electric what is the best way to develop stamina for fast playing
Get a teacher to work with you on proper technique. Doing things wrong on upright can cause long term pain. After that, practice, practice, practice ..
Proper technique is critical. You have to teach your hands proper hand position, and proper shifting. DON'T stretch your fingers like one is prone to do on electric bass. It's easy to injure yourself playing upright, and mean the kind of injuries that can become chronic. If you want to learn to play fast, practice slowly. Shifting accuracy, use a bow to develop tone; it helps your left hand tremendously. And the best way I've found to develop stamina is to play gigs for 3 or 4 hours. Find someone who needs an upright player for their piano trio gig, even if you don't want to solo very much. It will whip you into shape.
Find stuff you love to play and lose track of time while practicing. Do plenty of open string exercises to practice rhythm and bowing so you can make the most of your brain (before your left hand gets tired).