Bass excersises to improve stamina

Binaural

Has anyone got any excersises they can let me in on that would improve your stamina. I've been playing for three years now and I'm pretty good although I have noticed I cannot play complex or fast paced stuff for an extended amount of time, I can play anything I tackle with a bit of effort but I can't play such songs for any longer than twenty minutes. I plan on getting people I have been keeping an eye on to start jammin every few days and see if we can get something going however if I'm going to do that I am going to have to sort out my problem. Cany anyone help? I'm looking for general excersises or licks that would be useful.
BTW I play with fingers, just so you know.
Any help is much appreciated.





PEACE