Have you had a "fit" of any kind? Sorry if you said you did and I missed it. If not, go to a LBS and have them take a look. Knee pain like that is not normal. I would NOT be pushing the big ring.
In my case, I was 58 when I started bicycling. I put in 1100 miles the first 3 months, then did Ride the Rockies (400 miles of Colorado passes) on a mtn bike. I will be 74 in a month and never have had any kind of knee pain, period.
You really need to check out your fit.
Good luck.