You have underestimated the base fitness you have maintained in the last 15 yearsYou are probably spot on. In my twenties I was a gym instructor in the evenings and kept up a 5 or 6 day a week routine for many years. I've been a stay at home dad for over five years so I've gone from carrying a baby in a sling on long walks to school runs, park visits and active holidays. Being a stay at home dad is probably an oxymoron in that we were rarely at home. I've been lucky enough not to work for some time now so I'm not tied to a desk either.I think I took up running around 2002. I've had a phase of climbing and have a mountain bike. I guess putting all this together means that yes, I have maintained my fitness levels more than I realise.I started wearing a fitness band earlier in the year because I thought I was getting too sedentary. This has shown that I'm more active than I thought and whilst I'm sure it's not 100% accurate, it's giving me a consistent resting HR of about 47.So I'm far from a couch potato, but as I'm looking to the future and with the time I want to spend writing and studying, I want to make sure I don't slowly become one.On the idea of a marathon, I think of it as an arbitrary distance really. I've run 17 miles so maybe I will try 20 one day, then maybe 25. Of course it's a completely different kind of exercise than a quick 5 or 6 miles. Then I concentrate on keeping the intensity high and competing against my previous times. On a long run I'm just happy to do the distance and get home in one piece :-)Overall I see the exercise I do as enjoyable and even relaxing at times. It gives me thinking time and I often see an awful lot of wildlife.