The bigger question for learning to play guitar is "how hard do you want it to be?" Learning some songs can take a few minutes, while others can take much longer. (such as Stairway to Heaven)
In general, you should have no problem learning to play once you get calluses built up on your fingers. The initial pain from steel strings is what stops most people who try guitar. Having played two instruments for a few years, you probably already know you need to practice regularly, which will help you get past that quickly.

If you feel like side-stepping that part, you can use nylon strings, although you’ll get a much different sound out of them.
References :
5-years experience

Mentally it is not any harder than flute or piano. Physically it is harder because we are using the left hand to do most of the work and the left hand is not very coordinated. I strongly suggest taking a couple of months of lessons to get you started. This way you will learn the correct way to finger the chords and will have a teacher show you some of the shortcuts to learning. Once you have taken some lessons than it will be fairly easy to learn from the internet, etc. You can learn more in a month or two of lessons than you can in a year of trying to teach yourself
References :
G.I.T. graduate 30+ years playing/teaching
http://www.youtube.com/user/gtarczar