How to Build Muscle and Condition My Alaskan Malamute?

Lilion is my 2 year old female Alaskan Malamute, Purebred. When i go out on my quad she can not keep up and loses strength and speed quick unlike my other Dog Woo, Who is a Retriever X Aussie(designer dog) He has No conditioning or muscle building at all, and he can keep up with me for at least 4 hours to 8, starts to tire at 5-6 hours in. We go to the Pits to play on the quads. Woo can keep up but Lilion can go for 20 mins then tires out.We Love our dogs and want to help Lilion be healthy and to have fun with us. Is there any one out there who can give us some useful tips and techniques that can help. We know she is not meant for speed but for her to tire out so quickly is a sign to me she is not in good shape. Please Let Us Know what you think Please no snide remarks or comments. Our dogs are loved and cared for better then any animal out there. I want them to live their lives to the best & fullest, from Pampering and Babying to Proper Medical to Extreme Fun & Adventures!!!My 2 buddies are in my profile pic, DOBs, Woo 7/4/2010 ~ Lilion 8/8/2011 Thank You all ahead of time!

for her try going slower she was made to go at her own speed for a long time