I’m no expert in this area, but I will make two observations. Weight lifting is anaerobic, while mountain biking, skiing, surfing are much more aerobic. Muscle strength is important, sure, but it’s not about muscle mass or how much you can lift; it’s more about your endurance and ability to keep pedaling.
Second, my running coach shared a rule of thumb: it takes about as long to “lose” your training as it took to gain it. In other words, if you run a mile every day for three months, then stop running completely, it will take about three months for you to “lose” all the benefits of the prior three months of training. Again, this is for an aerobic activity, so for weight lifting it’s likely to be very different (as alluded by the guy who replied in the bodybuilding forum).
Finally, age is a factor. Sadly, as we age, the hormone levels in our body change. For men, testosterone typically drops. This can have a marked affect on the speed at which muscles will atrophy and the gains you’ve earned are “lost” by inactivity.
Best advice? Stay active using a variety of activities.