Hi MoiraRoseIsMyQueen,
A marathon next year is not out of the question; I would only ask ‘Why a marathon?’ If the aim is to get fit and healthy, there are lots of ways to do this that have a lower injury risk and just as much aerobic benefit. And you can have just as much, if not more, fun.
It sounds like your aim is to get into a habit of moving more and building up aerobic fitness. Typically, as mentioned in other channels, you do this in the following order: frequency, then duration, then intensity.
I’d build up the frequency by walking at a pace that is harder than your casual walk, but you can still speak full sentences. Get into a weekly (preferably daily) routine and slowly add time (duration). Once you’re doing this regularly, you can start to add some intensity. It doesn’t need to be running, it could be hiking up hills or walking with a weighted backpack.
At this point, you could start to do some longer-distance challenge walks. These are great fun, physically challenging and typically lower risk of injury from long-distance running.
Over months, your connective tissue will get stronger, and you may wish to start running or increasing aerobic volume via cycling.
C25K isn’t my personal favourite btw - technically, they rapidly get many people to go above their aerobic threshold (the point where talking full sentences becomes hard) very early on. For some, this is unpleasant and people start to drop out. I’d prefer to see a much slower ramp in fitness through brisk walking, hiking and cycling before moving over to running.
Overall, I'd approach the problem as how can I begin to love exercising and build it into a habit for the long-term.