... Sometimes I am dead keen and enthusiastic and do all kinds of stuff, but I rarely continue as I intend.... I'm not so youthful anymore ... And walking is achievable. ...
I feel I'm being successful in moving myself on from occasional walks to an almost-every-day-walk person and think two things have helped.
One was getting a fitness monitor. Literally all I use it for is the steps / distance measuring, and in 4 weeks I've doubled my average daily distance, just hanging on in there now at the 10,000 steps. I like the mindlessness of it - I just go off and walk, and check come the end as to whether I'm near the day's goal. That's made me walk round the block or round to the supermarket before now, for the bit extra which means I get the satisfaction of feeling the thing buzz when I hit the target!
The other has been to just plan for the walks. Most days, I'm now realising it's possible to fit the walks in, but I was very prone to getting too settled into other (sedentary) things. I still spend most of the rest of the time in sedentary things, but planning as seriously for the walking as the other things is a first for me.