When training, the body experiences damage. In the resting phase between training, the body heals and overcompensates for the damage done. Around the second day after a training session, you experience the most benefit from that single session. By the third day, your body begins detraining.
If you overtrain, your body will not be able to keep up with the load. If you undertrain, you will basically maintain your current level.
An ideal training schedule alternates different types of training on a two to three day rotation, with sufficient intensity to trigger the repair response, but not so intensely that you overwhelm the body's ability to recover.
Men over 40 sometimes experience low testosterone, which affects the drain ng response and general wellbeing. Doctors routinely test for this and prescribe steroid patches.
The body's healing processes slow with age, so it may take longer to experience full benefits. Look at your training regimen in the light of this background. The worst case scenario matching what you describe would be overtraining once a week. A better scenario would be to start at a more moderate level of training two to three times per week.