About a year ago I asked some of the fast guys over in the 50+ forum what kind of things I needed to do to get faster. (I'd ridden maybe as much as you have.) A rather astute person said "The first year just ride. Learn to love cycling. It takes 5 years to reach your potential." I rode as much as I could and worked on being more comfortable riding. In spin class and seeing other people riding bikes the most common thing I notice is low seat height and slow pedal cadence. Both are unnecessarily hard on knees. I think the trade offs are leg strength versus cardiovascular capacity. Slow pedaling stress your leg muscles and knee joints. Faster pedaling stresses you heart and lungs. I think improving cardiovascular capacity should be the first focus.
I'm not a big believer of signing up for "pledges". I do believe in consistency and cross training. If you feel like giving up then you're doing too much. It took me a month to get to being able to walk 20 minutes 3 times a week and I held myself to that for the first 6 months. What I was able to do over 2 1/2 years ago matters little. What does matter is that I stuck with it.
I would recommend finding that distance or time or activity that you're comfortable doing and just do that for another 3 months. Get to where you look foward to the activity then start pushing yourself just a little. Start thinking in term of how many years it'll take to get to where you want to be.