You only need three chords for rock and roll anyway 
 
Seriously, I suspect you'll find that having played keyboards will help tremendously in playing guitar. But perhaps more importantly, playing guitar will make you a better keyboard player. I started playing keyboards about 7 years after I started playing guitar, and it made me a much better guitar player. I would think the reverse is true as well.
 
Bottom line is you have nothing to lose by trying, and I think you'll have positive consequences for a variety of reasons...even if they weren't the ones you intended.
 
 

you didn't state your age... like said above, physical issues include arthritis, and older people like a friend of mine that plays guitar with me has issues with the thinning of skin as you approach your seventies.  I started guitar around 50, but my main prob was not having a strong musical background other than drums.  I picked up keyboards with the resulting education in music theory and that has helped my guitar playing, but struggle mentally remembering guitar chords if I get away from regular playing... i guess i spend more time improving on keyboard playing.  I can play and record guitar rhythm and leads for songs but it involves several takes and edits, but nothing like an experienced player can do.

My uncle started playing guitar when he retired, and got quite good in a short amount of time. He took lessons and and put in the time. I started a few years ago, I'm now in my 40's and the only thing holding me back has been me and too many other instruments on the go at once.
 
Having a piano knowledge should really help you just focus on the physical playing.

you have an advantage with having musical knowledge allready.
if you believe you can and you really want it, without knowing/realising it probably, just by believing/wanting it you actually instruct and programm your brain to forfill that task and it will be easier.
 
if somehow you find out later guitar playing isn't for you after all..
you could just hold the Strat to a Marshall amp and let it feedback and enjoy that on a saturday night..
personally I could just play with a Strat pickup selector all day and plan to do so when I retire.
 

The hardest instrument to learn is your first. After that, all the knowledge you've gained about music is applicable to other instruments you might decide to pick up. So in that sense you're already part way to being a guitarist.

Mexican-made Fender Strats are actually pretty good most of the time, and with a bit of modification can sometimes be made very good indeed. My only Strat is a MIM which I bought because it felt right and was acoustically the best sounding, most resonant and "alive" out of the nearly 30 I tried, including the US deluxe model of the time. A Mexican Strat is a perfectly good guitar to learn on, so long as it's set up properly. A good setup makes any guitar far more playable and they generally benefit from a setup once in a while even if they still retain the factory "standard" one.

The main thing in learning guitar, as with any other instrument or skill, is motivation. If you want to learn it you will. Even if you find you still need to "fake" guitar using a keyboard, playing guitar even a little will give you an insight into the chord inversions guitarists use and why we use them. They're often different to the ones keyboard players use, and in my opinion where many keyboard players fail at guitar-like sounds, riffs and phrasing is they play like a piano player, not a guitarist.

i don't think an "old dog" would have the dexterity in it's fingers paws, but an old person, . . . well, sure, as has been said by others if you have the motivation, the drive and the desire, then go for it, you already have prior musical knowledge to give you a jump star, don't waste any more time because as we get older time slips by all to fast, so just get in and do it. Be realistic, don't get depressed when you realize you don't match it with joe satriani etc, but don't limit yourself either, just be the best you can with the time you have available to devote to it, and most of all enjoy yourself 

Like the above responses, I also believe the more I explore other instruments, the more I seem to learn/absorb/
understand them all. So, being retired, I look forward to spending time on my keys, mandolins(ac +elec), lap steels( E with benders+C6th), banjo, violin, resonator, drum pads(no room for a kit yet), and an assortment of acoustic and electrics, everyday.
     When I learn a tune on one, I get inspired to try it on another and that really expands things for me! and it's fun!

The many comments above pretty much cover all bases brilliantly. A couple of things I'd like to add are...
 
Keep up the morale - if it gets too 'trying' then you're probably doing something that's a little too hard for you. Step by step gains that build the motivation that you currently enjoy.
 
Regular practice - a bit here and there may not get you where you want to be in the timescale you want to be there. From your keyboard skills you will know this, and that repetition is the way forward, at least initially.
 
Expect to become 'deformed' - the fingers will twist unusually, the segs will form on the tips, the wrist will ache, the cramps may set in... do not give up! Keep the goal in mind, not the pain and suffering endured en-route :-)
 
Stick with chords around the first three frets at first. Dabble with songs you know with these. Start to try bar chords after this (will need perseverance - few get them straight away). Let yourself/your hand get used to what's expected. Once you can play them (bar chords) it will open up a further world of possibilities.
 
I wish you all the very best with your endeavour which should fall into place nicely given your skills on the keyboard.

Hi. I do not have anything to offer that has not already been covered.
But I started playing drums at the age of 44, and have played a few gigs the last year or so.
As a guitar player, I feel like it helps me even with drums. I already had a "feel" for music, so I agree witht the people saying that your keys skills will only help you in your endeavour.
Most important of all (in my book), try to enjoy it Even when your wrist and fingers hurt
All the best.

Thanks so much to everyone for all the thoughtful and helpful replies. I have toyed w this occasionally over the years but between work and life (and wanting enough time on my piano and keys) I have never moved beyond the few chords I have known since I was a teen. 58 now and still working way too much but w kids launched or nearly launched, and northeast winter approaching, there is more free time for music now so I am going to jump into this!