I have DD's aged 5,10 & 12. They do still come out on walks with me, reluctantly at the start of each walk or adventure, but they soon forget that they aren't enjoying it. I try to make it a functional walk. Good tempts for me are:-
Borrowing a dog to walk - they love it and don't want to go home.
Accidentally leaving the car somewhere else and having to go and get it - usually unvolves me having a rare drunk night out beforehand (such commitment, I know)...
Walking to see someone I know they want to visit.
We also have challenges like having to get to a certain place but only being allowed to step on tarmac for 10 footsteps (requires them to plan the route and gets them trying to outdo each other to minimse footsteps.
Also we have a woods nearby where they have built swings and can climb treesso they do that as well, the older two at least. It is within screaming distnace of the house....
I also have a thing where we have to go somewhere we have never been before on a sunday. I live on an island 3 miles long so this is quite challenging! We have been in tunnels, up different paths. They also don't mind this as we actually can't go very far from where we know.
My DH also kicks them out during school hols when he thinks they have been inside too much - they hate it at first but get used to it.
A trampoline seems to have done the trick for the last year. Also they have guinea pigs outside and so they play with them - not so much in the winter,but a lot in the other seasons.
It helps that they know deep inside that they will probably have a good time in the end and so tolerate the start.
It is hard to prise them out of the door, though. I think in conclusion,a lot of this is about me joining in and leading the gang, but to be honest I need the exercise as well and it is nice to spend time together having a laugh as I work full time. I also like the fact that while they are not in the house, they are not making a mess. The crumbs will be eaten by the birds...