You could start by walking to the bus stop and walking one bus stop further every day, leaving earlier to incorporate the time needed.
Not sure what work you do but either walk up a floor before getting the lift or get out a floor before yours, do that everyday increasing the floors walked until you're walking rather than taking the lift! Then increase the pace up the stairs!
If you have a dog take him out for an hour or so walk every night, as you get used to it increase the pace.
If you're at home watching tv everytime an ad break comes on go upstairs. As you get better you can try for more trips before the programme restarts!
I'm sure I saw somewhere the tv exercise plan which was to do something everytime an ad break came on, like star jumps or running on the spot. You start when the break does and stop when the programme is on and you do it every ad break, by the end of a programme you might have had a mini workout of about 15/20 mins. Do this all night and it adds up to quite a bit lol.
I've swapped a chair at my computer for a gym ball and I sit and do ball exercises all night lol. Well for the most part i just bounce up and down lol with the odd proper one thrown in now and again lol
Good luck