Remotely Monitored, Mobile Health-supported, High Intensity Interval Training Before Hematopoietic Stem Cell Transplant

The goal of this clinical trial is to learn how a remotely monitored high-intensity interval training (REMM-HIIT) affects the cardiorespiratory fitness and physical function for patients planning to undergo stem cell transplantation. The main questions it aims to answer are: Is there a change in the participant's cardiorespiratory fitness level? Is there a change in the participant's physical function? Researchers will compare the REMM-HIIT program to a control group of participants who do not take part in the training program to see if REMM-HIIT helps improve stem cell transplantation outcomes. Participants will: * Complete cardiopulmonary exercise testing (CPET) 4 times during the study * Do basic tests to measure physical function 6 times during the study * Answer questions about their life and how they are feeling 6 times during the study * Wear a device to keep track of step counts and heart rate daily * Keep a log of every time they exercise throughout the study * Optionally, provide blood and stool samples 6 times during the study