Maybe some weight training? Increase muscle mass = stronger engine? You'd probably have to take some breaks to allow muscle to recover, but I think that would help. Maybe vary your distance? If you are biking 7-9 miles a day, maybe skip a couple commutes and go for a longer ride some other time? I remember a 5 mile ride used to be hard for me; I'd come home looking like I was about to pass out, and it took me 30 minutes! But I worked up kinda quickly (just the right amount of peer pressure from a local riding group) and in no time I could bike 70 miles! It was actually quite a jump- I started road riding in May last year, and before that the most I'd gone was 5 miles. I had been doing spin classes all winter, so I had a good cardio endurance base. Those rides in May were 14-15 miles or so. In June I was doing 18-20 miles, and at the urging of a friend who was even newer to riding than I was, I did 70 miles! She continued to increase and was doing centuries on weekend days. Oh, spin classes! If you can find a place to take them for cheap, they are very different from road riding- a lot of variety in rpm and resistance in a single class. Ooh, now I'm thinking you've plateaued because your riding lacks variety- just doing the same commute day in and day out.