My aim was to increase my average speed to 29-30 km/h on a 40 km route by the end of the summer before I jump on a road bike. I currently ride a light MTB on knobbies. The summer has almost ended but I'm still averaging about 26 km/h. And the most disappointing thing is, it has been the same since April. My endurance has increased a lot, but I just don't have the power to go faster.
What should I be focusing on while training to gain more muscle, or power?