I will be 63 in three weeks and have suffered bad knees since I was 19 from a horse injury. The left knee has had two surgeries, the second was a reconstruction 1977. The right knee is tired from years of compensating. I too just got back into biking with a REI Madrona Mtb and a Specialized Dolce Sport Compact 48cm. I use the Mountain bike for grocery trips and the road bike for group rides. I have *worked* to build up my leg muscles over the past two months to compensate for the weak knees, and can easily do a 15 mile ride. I am now looking forward to longer rides.
I also do exercises to build the muscles at home. A really good one is to get on the floor on my knees, feet behind me and lean back & hold.