being new to serious biking, I find that now (with everyone's help here) that food plays an amazing role in ability and energy. In 5 days, now that I switched from diet bread to whole grain bread, I went an extra 7 miles over the normal 20 and I felt great. I also switched from all water to gatoraide and wow what a difference riding now. I am trail riding this weekend a 40 mile trail and i cant wait.
3 weeks ago I felt like you did. I am still overweight by scale but in the last 3 weeks I lost about 5 inches off my waist line and now I have the engery to lose more.
Just a thought