I've had the same problem. I solved it by cycling to work. Forest Hill to Chelsea is a dream. It takes 1 hour, mostly back streets and cycle ways. Exercise for the day done, no need to pay for gym membership or transport. I started by doing it once or twice a week and now every day it's not raining heavily or windy I'm in my bike. Glorious. I'm an unfit almost 30 year old woman, no lycra, second hand bike. Highly recommend.