51 year old knee problem questions

I am a 51 year old male, who has run consistently for 33 years.  In the Marine Corps, I maxed all my PFT's for 14 years running sub 18 min 3 miles.  However, I badly injured my right leg in an accident in 1988, which altered my stride.  From there on, I started having knee problems.  My right knee felt weak and would be continually sore.  In 1998, I went to a sports medicine doctor who performed ortho surgery.  At that time, he rated my knee wear a 3 out of 4 (4 being the worst).  However, he encouraged me to keep running.  In April 2011, I ruptured my plantar fasciitis, which kept me out of running for a year.  During that time, I started doing more full body workouts, but noticed that I was really limited by my left knee.  For example, I can only do about a quarter squat before feeling pain.  When I go down stairs I unconsciously avoid putting weight on my right leg.  So I went back to my sports medicine doctor who did an MRI.  He stated that my knee now is a 4 out of 4.  His advice concerning working out was do not do anything that causes pain (i.e. deep squats).  Since I could not run anyway because the PF, I bought a roadbike in Feb 2012.  I started having a blast roadbiking.  I live right outside the Smokies so there is plenty of climbing, which did not seem to affect my knee.  In March, I started back running one time a week.  Through out the Spring and Summer, I primarily roadbiked.  The last triathlon that I did was in the summer of 1985, but for some reason I wanted to try one this year.  So I signed up for an Olympic Distance Triathlon for 24 Aug.  I continued to run only 1 time a week.  I was shooting for a sub-50 minute run in the 10K and was delighted to run a 42:34. I felt no pain on the run and no abnormal soreness the day after.  Now I have got the bug to train harder and get my time down.  

This is my dilemma.  

Question 1:  Do I punt running and tris in order to preserve longevity in my knees?  Or is there a way that I can train and still participate in Tris?  I do believe that I need at least a day rest in between running.  

Question 2:  Is there anyway that I can workout to increase my leg strength?  I was not blessed with big, fast twitch muscles.  I was very pleased with my bike split of 1:03:00 for the 40K, but I would like to improve.  I have been doing some quarter squats and leg presses.  I stop before the range that I know will cause pain.  I cannot do many of the plyo exercises because of pain.

Sorry for being long winded.  I would appreciate any advice or links that you might have.  Thanks!!!