ego centered motivation

I stopped biking. After 6 weeks of very diligent and regular 30-50 min of exercise, I can't find the will to do it again. 50 seems too long, even 40 minutes is too long, I can push impatiently for 20 and then go away. This is not the first time I see this, but before I couldn't even last 6 week. As soon as I finished the challenge all motivation went away. I'd hope that the 21 days to turn anything into a simple habit was would work, but it only did so partially. I tried to see this not as a challenge or an effort but more as a journey into perceiving my body, gently flirting on my limits to avoid burning but it failed. Now all I do is push ups, crunches, horse stance and air-boxing. Maybe I should reintegrate 20 minutes of bike before the muscles part. It would be a good warm up and cardio, improving the duration and efficiency of the other exercises anyway.