I help run another HealthUnlocked community called StepJockey (it's here: healthunlocked.com/stepjock... We're trying to get people to use the stairs wherever possible just to burn a few extra calories and get a bit of easy-to-do exercise so you can build up to more strenuous activity (stuff that might need Lycra and sweating!).
I hope you don't mind me posting here - I just found a statistic that I though might be of interest. Apparently, just two minutes extra stair climbing a day is enough to stop average weight gain in middle-age. There's more on the health benefits of climbing the stairs regularly on our website stepjockey.com/health-benef...
That probably equates to going up and down the stairs at a tube station - which, while not easy is probably cheaper than going to the gym!