The Prophet (peace be upon him) constantly preached the message of good health. He knew that healthy bodies carry healthy solis. The Prophet (pbuh) is reported to have said that a strong believer is better and more beloved to Allah than a weak one, and there is good in both. This shows the importance Islam places on spiritual and physical strength.
The Prophet (pbuh) once raced 'Aishah and she won. Some years later, they raced again but he won. This showed how the Prophet (pbuh) had maintained his fitness and the importance of regliar exercise.
Other ahadith shows that the Prophet (pbuh) had a naturally athletic lifestyle: riding horses and camels, working in the fields, markets (loading and unloading), even walking a mile or more through the desert to answer the call of nature, etc. So even going to the toilet involved significant exercise.
It is important to ask your GP or nurse about how to exercise before you start. It is recommended that gentle exercise for up to 30 minutes a day is extremely good - you should be a little out of breath and sweaty.
Good ways to start exercising gently
Walk to the shops rather than taking the bus
Leave the car at home for small trips and go for a walk
Get off the bus or train one or two stops earlier and walk the rest of the way
Instead of sitting at your desk in your lunch break go out and take a walk
Use the stairs instead of taking the lift