Exercise alone can't create the lean muscles you want or need to boost metabolism. The right amount of overall nutrition and specifically, protein, is a critical part. Less exercise (with more purpose) and more food (also with more purpose) make over 50 fitness achievable!

Freebie about muscle loss:

https://www.flippingfifty.com/get-lean-stay-lean-after-50/

Resources:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3354005/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3790587/

It takes Leucine: American Journal of Physiology - Endocrinology and Metabolism Published 1 August 2006 Vol. 291 no. 2, E381-E387 DOI:10.1152/ajpendo.00488.2005

https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/resistance-exercise-enhances-myofibrillar-protein-synthesis-with-graded-intakes-of-whey-protein-in-older-men/3D6DF3A0BD1E312A3BD39CB0A4361B0F

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5214805/

Feeding the Muscles You’ve Got or Want After 50: How Much Protein?