There are things you can take to increase exercise tolerance.

1) Acetyl-l-carnitine will help tremendously. Start at 500mg twice a day. Since many PN issues involve mitochondria, using carnitine may also help the PN. This is how it works: Normally mitochondria burn glucose--the result is lactic acid which can make muscles burn. Carnitine enables
the mitochondria to burn fatty acids instead, so more energy is available if glucose is gone, and the lactic acid buildup is less..or non existant.

2) Make sure you are getting enough B6--- this vitamin is used by muscles. Buy the P5P version for best results.

3) some people like creatine. But be careful with it. Start slowly. It always gave me muscle cramps.
You cannot become dehydrated on this supplement. Don't try it if you have any kidney issues.

4) if you have tight muscles that burn/ache, also make sure you are getting the RDA of 400mg a day of magnesium. Low magnesium impacts muscle function alot. See my magnesium thread on
Vitamin forum, page 1.