Everyone is different, as you can tell from reading everyones comments. I can only speak for myself, but I think its also knowing a bit about oneself as well. I'm 73 and I believe in the principle that the effort needs to be made! Like others hear I make SURE that I get out the house everyday, rain or shine as someone already wrote. If I start to falter my wife has instructions to kick me out of the door!
However, I don't tend to have this problem because I like to think I have one advantage, I like getting up early and look forward to each day. I also believe that on balance I'm lucky to have an optimistic personality (like my Mum, not like my Dad) and I don't suffer from mood swings and I don't indulge in negative introspection too much. I can of course and have done so, but I still know that even now I can enjoy certain life perks! Although I've had 7 hernia ops (cured for good now) and had my Prostate removed because of cancer I am, in 2018 still basically a healthy old bloke and I still make the effort, but more importantly its because I WANT to make the effort!
I walk every day, I have a number of routes and I rotate and use them all year round. I also like to run off road which when I'm home I prefer to walking, but I had an injury earlier in the year that lasted for ages which stopped my running dead! I'm now back to normal and plan to start running again in September, but after I finish my August hikes.
I do however dread what I will be like in my mind when I can't do any of this, but that's for another day.
All I can suggest is for you to fight what your mind is telling you, get out there, make the effort and I'm sure you will be the better person for it. It may take time, so give it time. Do not expect instant success, give yourself a time limit, for example 6 months. Do your stuff, get to that date and then compare and contrast as to how you felt at the beginning and how you feel 6 months later. I bet you will then feel positive about what you have actually achieved!