I turned 52 in September. Several years ago I picked up the "Body for Life" book and did a 12 week "challenge" - cleaning up my eating and doing their recommended mix of weights and aero. I lost 12 pounds. But the 6X/week exercise sure left me feeling drained. I'm still waiting for exercise to energize me, instead of just making me tired.
I did continue lifting after that, going to a workout routine that was a bit shorter on the reps but with heavier weights, but I find the aero to be horribly boring so I slacked off on that. I did get a brisk 20 minute walk in most every day at lunch while at work though. The eating control was slacked off too. I gained the 12 pounds back, but there was noticeably more muscle in my body than when I started exercising. I'm a couple of pounds down in my near 2.5 months of retirement, but I'm not eating my 6 small and monitored meals per day.
I continued doing weights 3X/week at home (I've never gone to a gym - that's too much extra effort and so would be easy to avoid, while having the weights in the garage eliminates one excuse) until the beginning of this year when I developed a mild-moderate pain above one elbow with certain motions. I cut the weights out hoping things would get better, but it looks like it is going to be chronic, even after doing some exercises given to me by the physical therapist at the clinic.
When I retired at the end of August I decided to start making myself go out for an aerobic walk (and I walk quite briskly at all times) for 40 minutes every other day. That started out at approx 3 miles in the 40 minutes. A few weeks ago I started trying to jog, and I'm now doing 4.5 miles in the 40 minutes. BUT, I've had to drop back to two days of rest between each session, or my distance covered drops noticeably (about a minute's worth over the 40 minutes). It seems that I just don't recover that quickly. Depending on how my hip and knee joints hold up I may have to alternate jogging sessions with aero walking. A little sciatica-type nerve pain at night after the first jogging session seems to be avoided by doing a set of straight leg dead lifts with 90 lbf.
Something I found with the BoL program (and while it may not be the "best" program, it is A program, and if people will just follow most ANY exercise/diet program they'll see some positive results) was the importance of keeping records of your workouts. If I wasn't timing my jogging sessions and noting the distance I wouldn't be able to spot that the more frequent sessions were causing me to lose ground. Keep your records, analyze what they tell you, and pay attention to how you feel.
My garage cleanup needs a bit more work before my weight bench is uncovered, but when it is I'll start back in with the weights. 30 minutes 3x/weekly isn't too onerous (though I do not enjoy it, I can usually force myself to do it).
I think you are best off if you have a recreational activity that gives you the exercise, rather than exercising for the sake of exercising. But unfortunately there isn't much opportunity to ride dirt bikes here in San Francisco, so I'm stuck with lifting weights and walking or running. Still, 4.5 miles in 40 minutes is a world record for me for both distance and time, so I have some some satisfaction from not letting myself wimp out, though I usually start whining to myself after the first block of jogging.
cheers,
Michael