Of all the advice around about keeping fit and healthy I think one of the better ideas is to try to increase the number of steps you take each day with an ideal target of 10,000. I've studied this a bit and can tell you that you can do about 100 steps in a minute if you walk at a reasonable pace so it would take 1 hour and 40 minutes to do the 10,000 in one go, though that's unlikely to be necessary unless you spend all the rest of your time lying on a divan eating peeled grapes.
I do a useful and consistent 2,000 steps 9 to 5 in the office (without going out), where I spend most of my time sitting and only occasionally getting up to fetch something or to go and talk to somebody - so the steps mount up almost imperceptibly. I guess the average person will do quite a bit pottering around at home too (although I confess I'm probably closer to the grape-eater in my domestic setting). Nevertheless I suspect that most people doing sedentary jobs are not going to do more than 5,000 a day without contributing purposefully by going for a walk (although if you use public transport you may well add a lot getting to and fro from bus-stops and so on).
But it's hard in the winter to get out for a walk other than the weekend (my pictures are from a 6,000 step walk late afternoon today - the second picture taken just 14 minutes after the first). I try to compensate by going to the gym and if I was more consistent with that I could hit the target with the added bonus of some affect on cardiovascular fitness and suppleness. Walking out lunch-time is a good idea for office-based staff especially in winter - and surely good for our heads too.
I am wary of giving anybody advice on exercise but will just offer the suggestion of buying and experimenting with a pedometer (that's how I checked my own step-count as described above). Unlike most exercise equipment it costs very little and needn't depress you and sap your will through lack of use; rather it can actually show you the good you are already doing yourself and encourage you to increase activity in your own time by taking the odd walk opportunistically. If you do get one don't set unrealistic targets - just monitor your current steps for a while and try out new habits to increase them. I find the psychology which goes with the pedometer most encouraging, for example turning walking around the shops from a chore into an opportunity to exercise. Try my Saturday shopping habit of parking free outside the town centre and walking in - lots more steps and save yourself a couple of pounds to buy a nice lunch.
