If you aren't riding a bike professionally

You are doing yourself a disservice if you train in the off season just for your hobby.

Your hobby will give you the exercise you need to do your hobby, but the muscular atrophy you're experiencing as you age will hit you hard in your golden years.

Pick up heavy things.

or, 50 is the new 30

But to your point I see it on almost a daily basis, both those who have benefited from strenuous strength training and those who are suffering from a lack of it. There are a couple of "old guys" that come into the gym on a regular basis who are impressive and one guy in particular who is pushing 90 and still holding it together. He doesn't lift very heavy, but does combos of things like squats to an overhead press with dumbbells and things like that are probably a large part of the reason why he still walking and can climb a flight of stairs.

Weight lifting is one of the best things you can do. Like has been said, you don't have to go heavy.

Screw cowering indoors lifting things all winter. 

Go out in the woods and lift logs and boulders. Put in some reps on a come-along.  Swing a double-bit axe.  Move a mountain one shovel load at a time...

You're projecting WeTYC.

I was snowboarding with a 75yr old man earlier this week.

Posted by: ReductiMat

You're projecting WeTYC.

You're absolutely right, I was contemplating my tasks for last Friday and Saturday and calculating the future benefits to my age-atrophying muscles.  I'll be back at it tomorrow--I probably shouldn't go into detail about which part of my anatomy is jutting out with anticipation of a day of trailwork-out.

It never occurred to me to do a cost-benefit analysis of having muscles as I advanced in years.  They've always seemed so handy.