I am also 68 and after an 8 yr. layoff I got back on the bike at age 59. So, I've been back riding for 9 yrs. now. To begin with it seems to me you are doing some good stuff in your training. When I started riding again I was 195 lbs and at 5'9" that's significantly overweight for a cyclist. You may not face the same issue but it was big for me. What I found is that losing weight and just putting in miles is the starting place for training. Last year I broke my neck and had my spine fused. In the process I got down to 180 lbs and then had my best year of cycling. I was faster and stronger. Once your body is fit I think learning technique and strategies for training......like LSD, intervals and maybe some TTs will be helpful. Seems to me you are on the right track.