10 weeks out from total knee replacement. I was told that the healing process could take up to a year. I'm concerned about fatigue and lack of energy?

Expected symptoms. Total knee replacement is a major surgical procedure and it takes time to recover. Sleep habits are disrupted for 2-3months, after effects of anesthesia, pain medication, and decreased activity level cause fatigue and a "lack of energy" in most patients having this surgery .. While the knee may improve for up to a year or more, most people have regained their energy by 3-6 months.
Optimize health . A total knee replacement is a big surgery. Several things can contribute to fatigue and lethargy. Anything from disruption of sleep patterns secondary to pain, blood loss from surgery resulting in a relative anemia, or poor diet can result in lethargy and fatigue after surgery. A discussion with your surgeon can often help assess common post-operative causes of fatigue and lethargy.
Normal. You just underwent the biggest operation that your knee could have. This takes a toll on your body and will take you time to recover. Need to stay motivated and keep walking and sticking with your range of motion routine. Your goal should be full extension and 120 degrees of flexion which is average. More is better w/ flexion. It will get better.