Walking, Swimming and Cycling are the three most effective ways to reduce weight. 30 to 40 minutes a day along with diet control will help reduce weight fast and safely.
Apart from this meet a general practitioner to make sure you do not have any medical reason to put on weight (
Thyroid function).
Are you stretching your work time? or stretching your muscle during work time?. Any back pain if it persists then it is better to take rest and consult Doctor or Physiotherapist.