Hey Petal. I get into a good routine with running and then I fall out of it again. I have a fairly good baseline of being able to run for 30 minutes even if I haven't gone in a couple of months, probably just from the certain amount of walking I do and the other gym classes/yoga/swims I fit in. I think I have just consistently trained enough over the last 4 and a half years that I can slack off for a bit but it doesn't take a lot for me to get back into my routine again. My baseline fitness is so much higher than it was 6 years ago, it's ridiculous.
If anyone ever asked me about exercise/running, I would say that consistency is key. My housemate could eat a ton of Mexican food and drink his own body weight in booze at the weekend, but he had such a fit body with low body fat because he was consistently training every week for years and years of his life so far. Obviously, that gets harder to maintain as you get older, but I always think that routine is king when it comes to health.
Long story short - keep going with the walking challenge and you will reap the benefits.