I started commuting in March. I bought a 1/2 decent hybrid, planning to ride to work at least once, maybe twice a week. I have 2 young 'uns, so I didn't really have time in the evening for sports (knackered my knee playing squash, so competitive sports were out anyway). I ride 20 miles daily, and timewise I set off 25 mins early, and get home 25 mins later, so really it only costs me 25 mins in the evening where I'd see the kids.
I really caught the cycling bug, and now I commute 5 days out of 5 apart from the odd occasion where my knee's playing up, it's very windy, or I need the motorbike or car to go out after work. My knee is massively improved.....I've lost 1/2 stone, have legs like tree trunks, and I feel almost as fit as when I was in my 20s!
I've just ordered an electric bike for the odd windy/dodgy knee day, so I should be able to manage EVERY day. I figure it's better to have a bit of help and still cycle than just take the motorbike/car.