When I take my adult dog out to the yard, and lead him around on a long line, he will sniff for a half hour before going potty. As my mobility decreases, I need him to potty sooner and of course, i don't want to stand out in the rain or snow either. How can I change this?
It sounds like he has learned to withhold his potty events until he's ready to go in. This is common when the outdoor time is not reliant on him pottying first. In other words, he needs to potty first and go sniff outside after after as the reward.
To change this, you will need to choose a new potty location in your yard. For example a 15 foot square defined area in your front yard rather than the back yard where he has been previous doing this behavior sniffing. (Changing the environment changes the behavior expectations). Don't worry this physical location change is only temporary until he understands the new way to do the behavior.
Starting first thing in the morning when you know he has to go pee, get dressed as if you are going for a walk, take him out on a leash (not a long line) and stand in the middle of the defined area and wait until he goes. Let him move and sniff around only in the defined area. It helps to anchor yourself on the spot. You can turn around on the spot but not step away. A six foot leash actually give him about a 15 foot radius which is plenty of space to explore to potty.
As soon as you get out there, set your phone timer or watch for 5 minutes. If he does not go in 5 minutes, then bring him back in and confine him to a small area in the house (X-pen or crate for example). Take him out again in an hour and repeat. Repeat for as many times as it takes for him to go in that 5 minute period. When he does, wait until he's done, then mark and praise him and take him for a long sniffy walk - at least his 30 minutes. If you repeat this each time you take him out (at least 4 times each day for most dogs-after meals, after training, play or sleeping, and before bed), he will start to understand that he must go potty first, then the play and walk comes afterward.
Once he is reliably going potty right away in the new location (10 times in a row), then you can start to take him back towards the previous location where he was pottying. The same rules apply in all locations of your yard now, including the the old location once you get there. Take him out, if he doesn't go in 5 minutes, back in the house and try again in an hour in the same location. If he does go, he gets a walk or playtime afterward. Later you can phase this into a yard or in-house training session if he enjoys those. What comes after the potty must be rewarding for him. Over time, you can shorten the activity or only go for a walk once or twice a day.
Since you are already out there with him, you can also add a cue to the potty and teach him to potty under "stimulus control" so later, you can cue him to go whenever and wherever you need him to.
While you train, make sure he is getting enough mental and physical stimulation each day. Do indoor training sessions, or ask a friend or hire a dog walker to supplement his walks 3 times a week. Good luck!