Patrick

Hitting the wall is very common among us shortly after treatment and can be due to an accummalation of things. You have to remember that your body is still healing and is in a hypermetabolic state and will burn through calories in a major way. Your intake should still remain high in calories with an emphasis on protiens, vitamin A, C and the mineral Zinc (as suggested by the Cleveland Clinic).

Thyroid issues are common after rads as David pointed out so get your TSH levels checked. You may want to also have your adrenals and testosterone levels tested as well. Opiate pain medications(major cause), antidepressants (major cause), stress, poor diet and exercise all will effect testosterone levels which will also effect your "get up and go". Being in your late 50's is against you there as well as the typical male loses 1-2% of testosterone production after 40 anyways. Ask your Dr about getting tested and possible testosterone replacement therapy.

Drugs will also do a number on you, pain meds especially as they depress your central nervous system, lowering your cardiovascular system, making physical activity harder while constraining testosterone production. If the issues persist make sure you are asking questions and seeking solutions as the squeeky wheel gets the oil.

Keep your chin up

Eric