Some good suggestions so far. There is a way to do it and actually see progress and measure what areas of fitness you need to work oout.
I have climbed tons of 5.12’s on lead outdoors and in. I have bouldered as hard as v 9 outside (with exception for falling on the last move of a v10 years ago.) I have climbed 5.13’s and have not climbed for the past two years.
Here is how you CAN climb 5.12
Cardio: Increasing body efficiency through cardio excercise will help you with endurance in training and actual climbing. Climbing demands all around physical fitness. Start doing 20-30 mins cardio 3x per week. Increase time to 45min after two weeks. Work your way up to 1 1/5 hrs.
Abs: Climbing hard means your core muscles need to be very strong. V-Ups (crunches combined with leg extensions) are great. Ab wheels, sit-ups. Check out youtube for interesting abdominal workouts. Train abs EVERYDAY! A good drill to do in the gym:
Find two decent handholds on the steepest wall you can find. Practice picking out specific footholds within reach and precisely toe point onto each hold. Try this for predetermined time period (30 seconds to start.) Make sure foot holds are all over the place. Good practice for keeping your feet on the wall.
Push-Ups: Great to keep opposing muscle groups fit. Vary push-Ups. Change hand positions (wide, narrow, Triangle). Try and balance on medicine balls, etc… Great at building stablizing muscles too. (very important)
Pull-Ups/Lockoffs: This may seem obvious but you want to work on being able to hold a lock off for 60 seconds or more. The next goal should be to be able to do a one arm lock off. One arm pull ups are different and arbitrary for this goal. The ability to lock off on progressively worse hand holds will be a great tool in your climbing arsenal!
Climbing technique: When warming up climb silently. No bashing shoes against the wall (if indoors). Be very precise with your foot placements. Learn technique if you don’t know it. Another good tool is to climb with your arms as straight as possible when warming up. Try and climb the route without pulling at all (forces you to think about body positioning and footwork.) To hone your footwork climb lots of vertical routes with small footholds. Learn how to smear. Learn how to backflag, drop knee, etc…
Stretching is also vital. Learn good climbing stretching techniques. Make stretching a habit. Great at preventing injury as well as promoting flexibility.
Develop your power through bouldering. Learn how to move dynamically as well. Dyno’s, campusing, etc… Set reasonable goals. (I.e. if you can climb v5 consistently you can climb through 5.12 cruxes.)
Finger strength is key. When you are not climbing, squeeze a tennis ball through out the day. Grabbing sand is great too. (fill a 5 Gal bucket with sand/rice. Work your hands into the bucket by opening and closing your hands.) WHen you can successfully do 3×10 sets of pull ups without much effort, try and use a hang board and different grip positions to strengthen hands.
Attitude. Visualize success. If you have fear of fall for instance. Climb very easy routes on lead (assuming you are properly trained and can make good judgement decisions about your climbing ability. You accept all consequeces for your actions.) And skip a bolt every now and then. Pick a route suited to your style. Climb it obsessively. In your mind visualize success. Learn every ripple of rock. The feel of every handhold. Breathe. Mark every foothold in your mind.
When you complete this climb you will be stronger. Physically and mentally. 5.12 is really only a mental barrier with a few physical challenges. Once you break through the challenge, 5.12 and beyond seems achievable.