Pain on outlide of left knee after ~9 miles

I have been running 3-6 miles on a sporadic basis (i.e. on & off training) for about 15 years now.  I have resumed  regular running over the past month or so, doing a couple of 5k runs per week through hilly terrain or in the gym track doing intervals.  No issues at all.  I did my 1st sprint tri last Saturday, and it went great.

Today I went running with my wife, who is training for a 1/2 marathon, the original goal was ~8.5 miles, but I was feeling good, and figured I'd push for another few miles.  I ended up doing 11 or so in total.  My pace was probably around 9:45 or so, had plenty of reserve in the legs, and breathing was fine.

Around mile 9 or so, I start getting this pain in the left outside of my knee.  It comes and goes, but by mile 10, I had to start mixing up walking and running as the pain got pretty bad.  There was no trauma i.e. slip or lost footing.  One key observation is the paved trail/road had a pitch to it, and I think that was causing the flare up.  By the end, I was at about a 11-12 min. pace, and my strides were turning into a shuffle. 

My conclusion is that I went beyond the capacity of my tendons & ligaments which caused the discomfort.  Logic would suggest that going from ~35 min runs in the hills to a 130 minute run on a pitched road would be a shock to my body. 

I am considering doing the 1/2 marathon with her, which is in 9 weeks.  But - I don't want to hurt myself by being gung-ho in the process.  

I would suspect that if I get a few 8-mile runs in on some variable terrain it will allow me to get past the threshold I was at today.  I think the obstacle is getting my tendons & ligaments accustomed to the work. 

If you have experience with anything like this I'd be interested in hearing.  Thanks...!