Some thoughts -
I can relate to the question because although I have been playing the guitar pretty much continually since I was 14, about 4 years ago I decided I wanted to learn to play the violin…I mean SERIOUSLY play the violin! My goal was to play the Bach Chaconne in 5 years. Never touched a violin in my life prior to that.
I was lucky to have an excellent teacher who ended up teaching me as much about playing the guitar as he did the violin because, as others have said, learning to play anything has the same set of issues.
First, the old saying "Practice makes perfect" should better be phrased, "practice makes permanent." So it's critical that practice be useful and that whatever you are practicing be practiced correctly no matter HOW SLOOOOOW you have to play it. Always play the passage involved as slow as you need to to play each note correctly BUT at the correct relative tempo. IOW, don't play at normal tempo and then slow down to play those several tough notes and then speed back up. Doing so is a perfect example of practicing incorrectly and making it permanently wrong. It's far easier to learn it correctly and slowly in the first place than it is to correct something.
Another useful item. though it may not apply too much at first, is don't waste practice time on something you do well. It's easy to have a problem with some part, get annoyed and then just go play something that you already can. My violin teacher jumped on that really fast one day - "Why are you starting from there? (6-8 measures before the hard part) We already know you can play that. Start on the hard part!" I really learned to concentrate on the hard part and surprise, that formerly hardest part of the piece became easy. Up till then, even on guitar, if I got to a rough spot I would go back and start over, thus actually spending relatively little time on the tough bits. Seems dumb now but it's what I did.
When I was playing flamenco guitar regularly in NYC, I often practiced 6-8 hours a day. I wondered why, in some cases, with all that practice, why certain passages remained difficult. I learned in my violin studies that I was approaching it all wrong. First off, 6-8 hours of practice is way beyond the amount of practice that is useful. It became "rote" practice, not actual THINKNG about what I was doing. So now although I may play for a lot more than a couple of hours on a given day, I don't PRACTICE any longer than 2 hours - and basically 45-50 minutes, a break, and then 45-50 again.
As far as learning the fretboard/playing with music, I'd say, as others have, that it depends on what you want to do. If you want to be able to play guitar chord-type accompaniment for singing, I'd say it's not necessary though you may decide you'd like to learn to read music for guitar. I can sight-read music for piano and violin but I can't for guitar and have personally never seen any reason for me to do it. It doesn't add any capability that I need for what I do and enjoy. But again, that's totally a personal decision and is affected by what you want to play.
I'm a big believer in chord-based application. Once you know the chords in any key, you can basically play/accompany anything that you can hear because any tune/harmony is made up of those chords. Obviously the more keys you know, the more flexibility you have but you can get by with a few of the common guitar keys if you are ok with using a capo to accompany someone who is playing in a different key.
If you are planning to accompany yourself or someone else, then your choice of keys to learn would obviously based on what key you/they sing in. Again, you could probably learn only one key and, with a capo, move it anywhere you need it but often guitar keys are chosen for the specific notes/registers available for a specific sound. IOW, you could play C-position chords capoed up 4 frets and be playing in "E." But the sound of the guitar will be very different than it will be if played using E position chords.
But, as previously mentioned, the guitar can be played a ton of different ways. I once heard a guitar teacher explain to a student that Paco De Lucia uses improper technique; I'm pretty sure Jimi Hendrix did too…
