I'll give a slightly different perspective. I started in the same place: pretty good guitar player, bored with my playing and newly interested in fiddle music. Added (not all at the same time) piano (wanted to learn about Music and thought that was the best vehicle), mandolin, violin. And yes, penny whistle.
Fast-forward a few years. I was going back and forth between mandolin and fiddle, playing out in a couple of bands, one Irish, one blue grass. My ability was broad but shallow. I decided if I wanted to be more than average playing anything, I'd be more likely to make progress if I focused my efforts. I decided to concentrate on mandolin. Mandolin and violin ought to be a good fit, but the mechanics of playing them is totally different, and you need a totally different approach to playing the same tunes, in my opinion. It's tough to serve to masters. At least it is for me.
It's difficult to be objective about how much progress you make, but I would say I am probably twice as solid as a mandolin player today I was a year ago, when I decided to put most of my effort into mandolin. I'm not sure the typical listener would notice, but I do, and my bandmates do.
That said, dedicated time each day to:
* Scales and arpeggios with a metronome (working out of Flesch)
* Bach (I have yet to get one down enough to play it in public, but I'm close)_
* Things I know (lots of Monroe, American and Irish fiddle music)
* New tunes and songs (mostly blue grass and OT these days)
"Practice every time you get a chance." -- Bill Monroe
"Style is based on limitation." -- John Hartford
'21 A Jr.; '24 Snakehead; '12 Duff F-5; '13 Black A2Z repro; '82 D-35; Nikes; that watch that keeps getting mentioned in the Gibson/Nike thread