Take it easy for the last part of the ride and just let the feet follow the pedals around. This prevents the muscles getting stiff for the rest of the day. Maybe you are riding in a lower gear because your legs like to pedal at a higher rpm - thats a good thing. Its good training to vary your pace - try sprints between certain traffic lights and relax between others. For reach and bar height you want to have your weight spread between your feet seat and hands. If your arms and shoulders are getting tired, try doing push-ups - it wont make you a more powerful rider, just more comfortable.