Fatigue easily on my afternoon runs only

I am 59 years old and have been lifting weights (2x a week) and jogging (5x a week) since I was in my mid 20's. I have noticed in the past few years I can hardly complete a 1/2 mile in the afternoon runs, but when I go to the gym the following morning and on my other morning runs, I can do 20 minutes (2 miles) on the stair climber followed immediately by a 20 minute run (2 miles) on the treadmill and 90 minutes of weights.
The afternoon runs are attempted after all day at my work computer. I also take a beta blocker once a day. My afternoon symptoms during my run are not breathlessness but rather muscle fatigue(leg, neck and lower back).
Nothing has changed over the years except age and the addition of the beta blocker. I am wondering if it is the timing of taking the bb (after am workouts), aging process, diet or ?

thanks for your thoughts

I think it's age. It's normal that our bodies get less strong and the things we used to do will slowly become harder even when keeping up the training pattern. But, in your case I think it might be because when you go to the gym you are obviously warming up first before starting to exercise. If you'd do some warming up exercises before the afternoon run do you think that could help?

That beta blocker will definitely slow you down as the day wears on. I found that on my original dose that I'd get through dinner and be wiped out physically. No problems staying awake 'till 11pm or later, just don't ask me to do much more than walk to the bathroom! Had the dose reduced a couple months ago because I was doing well, and now I can physically go until about 9pm which is a vast improvement. My BP is still good with the lower dose, so I'm lucky.

As ever, talk to your doctor. Try to schedule your exercise on that basis. Remember too that you need exercise 4 or 5 times a week, not necessarily 7! If you feel that you can't and the exercise is too important to you ask your doctor about maybe a reduction in the Beta Blocker dosage. Sometimes a life quality decision is more important than a speculative longevity one. If he agrees, he'll probably ask you to monitor your BP and pulse rate more closely to verify there are no harmful effects.

Bottom line ... talk to your doctor!