Been banging on a guitar for over 50 years and can rise almost to the silver rim of mediocrity even by today's standards with a 6 string.
 
Bass however another monster. Have had to do bass in various sessions over the years and has always been a physical challenge for me on several levels.
 
Have a 60's Fender Jazz which is a complete burnout to complete a song in one piece. Got also an Ibanez SDGR. Great sound and easier to play but still can't get thru a 3 minute song without falling apart.
 
Then the curse of a guitar player trying to play a bass and not screw up everything because the mental approach is still playing a 6 string, and not remotely in tune with the concept of what it takes to play a bass correctly.
 
Did a repair on an ancient Gibson EBO bass with a neck that was thin and almost as easy to navigate thru as a guitar. Joy to play but don't think they make these any more.
 
Any advice?
 
Fender used to make a 6 string bass.
 
Any alternatives to fighting with massive strings a mile apart. 4 string, 6 string variety.
 
Practicing with the Ibanez, but a clear reminder that playing a bass ain't for wimps. Spotlight is always on the lead guy, but the bass man is way cool and an unsung hero in the band.
 
John

Play the 4 string. Bass is entirely a different function than guitar and as long as you relate it to a guitar layout the longer it will take to produce functioning bass parts. It's a completely different frame of mind so
  I say this as an aspiring want -a- be bass player that just plays guitar too. 
good luck!

BayouBill is right.  It's a completely different beast.
I played 6-string for around 15 years before picking up a bass, and went through the same roadblocks.
 
For me, here's what helped:
1.  Play with your fingers unless you really need the percussive attack of a pick.  Since I pick a 6-string, this helped to separate the two.
2.  Remember that the bass' role (in most rock/pop bands, anyway) is to be part of the foundation, linking the drums with the other instruments.  There's no room for flash, and while a guitar can push/pull the beat on a whim, the bass should lock tight.
3.  Learn a lot of Motown & Stax/Volt songs.  
4.  Practice a lot.  Not so much for the notes, but to get the stretch and strength.
5.  Wave goodbye to those groupies you had as a flashy 6-string guitarist.
 

+1 on Slugbaby's recommendation on playing with your fingers. The way you need to hold your fingers and mute strings puts you in a different frame of mind right out of the gate.  I would also incorporate some simple slap and thumping, get yourself tuned into a groove psychology.  
 
I went 5 string and I absolutely love it.  I don't see ever going to a 6, just not interested in playing notes quite that high on a bass.  But that low 5th string sure is nice to get under those 6 string guitars no matter where they're playing.  
 
 

Guys,
Appreciate the feedback. Stretching, hand strengthening, practice, practice...and try to break the pick habit.
Any recommendations for a smaller closer neck also welcomed. The old Gibson bass was proof that it can be done. Not familiar with what is out there these days.
 
John

Great tips here and will definitely revisit this once I get my first (starter) bass (waiting for my tax refund ).
 
I'm thinking of getting the Ibanez GSRM20 short scale bass (John not sure if that's what you're talking about "smaller close neck")......my little boy will also be able to play it.
 
 

The only respect a bass player gets is when the drummer tells him he's gonna steal his woman and sleep on his couch with her.

There's a lot of short-scale basses out there that should help.
https://reverb.com/ca/marketplace/bass-guitars/short-scale
 

Gibson still make the EB0, only nowadays they call it the SG Bass.
http://www.gibson.com/products/electric-guitars/bass/gibson-usa/sg-standard-bass.aspx#disqus_thread
 
Also available in a "faded" finish (no top coat of clear lacquer) for less money.
 
Epiphone, Fender (and Squier), Ibanez, Gretsch, Hofner and others also make short scale basses.
 
Of course, a shorter scale does mean using heavier strings, or "standard" gauges with much less string tension.