Take it easy!
Take it easy on the water fast.
Walking is OK.
Running is not OK.
Running can drain you for too much energy ... making you sick.
Listen to your own body ...
Water Fasting and heavy weight lifting or running up the stairs are not very compatible activities ... you risk a serious damage to your kidneys!
If you want to exercise ... juice fasting is more appropriate.
To continue loosing weight after the fast ... you must change your diet.
Foods like fruits, vegetables, fish, nuts, lean meat are less likely to make you gain weight.
Foods like grains, bread, cheese, sugar, butter, milk, oils, chocolate, fried foods, pork, wine, beer, industrially processed foods, sodas etc are very much more likely to make you gain weight.
It is very difficult to gain weight if you eat fish, vegetables, fruits, nuts and some lean meat.
WS