Slowing Down Atrophy

Last week, I went to the gym on Monday and Tuesday, stayed home on Wednesday, and returned to the gym on Thursday and Friday and on Saturday rode our stationary bike downstairs for 50 minutes and did nothing on Sunday...  my exercise routine at the gym is 20-30 on the bike, 10-30 reps on 5-6 strength machines using no resistance to 40 pounds that target my back and shoulders and conclude with 20 minutes on treadmill at a speed of 2.5 which is relatively slow...  I want to get it up to 3.5 eventually or even 4.0.

Depending upon how fast I peddle and my time of 20-30 minutes, I burn about 150-180 calories...  My calorie intake now is 1500 to 1800 each day with the understanding that I could exceed that amount but hopefully not going over 2000.

This week, my plans are the same except that I will not take Wednesday off but may take Saturday and Sunday off instead.  I want to make sure that I do not push my body too hard and strain some of my muscles which would cause me to take off several days until they mend.  I also don't want to build muscle just tone and strength the muscle that I already have or at least slow down the atrophy.  At 73, I am hoping that I can keep up this conditioning until I am 83 or beyond.