“What are the physical fitness requirements to serve a mission?” Liahona, Sept. 2013, 55
Serious physical challenges or medical limitations can prevent some people from performing missionary duties effectively and can negatively affect their companions’ service as well. Such people do not normally serve missions. For example, those who need a wheelchair or crutches or cannot perform daily tasks without the aid of others are not recommended for missionary service. And being significantly overweight can cause a person to have serious difficulty with the daily rigors of a mission, so priesthood leaders may ask some people to lose weight before they recommend them for service. Regarding general fitness, one guideline might be that you should be able to walk about six miles (9 km) a day without difficulty.
If you have any questions about any of these matters, your bishop or branch president can give you answers and guidance.