My bass playing was stagnant since 1985 after the army , also I did not touch my bass for 20 yrs. I started playing the guitar again last dec, I learn to do some lead guitar, learn the different scales and after a few months I can do some solo and improvisation. I found that my bass playing has improved a lot too in terms bass runs and improvisation.
You need to know more music theory in order to improvise and do bass run , learning tabs will not help you , anyway most of the tabs on the net are not correct . I still prefer to use my ears to get the notes for any song, I think it is important to know the chords structure for any song in order to do your bass run . Don't limit yourself to one genre of music , listen to differnt genre of music, jazz and funk gives you the best grooves.
I grew up in the disco era , lots of great bass groove from that era and I think groove is something you have to feel for yourself , can't be taught by someone.
cheers
purplehaze3691