I was in exactly the same position back in March 2010 ... not been on a bike for 18 years, doctor telling me to "take some exercise to help my asthma".
My experiences:
Starting with a raleigh banana mountain bike (retrieved from the compost heap), riding 2 miles to/from work a day.
Then cycling to my lock up 6 miles from home, to get the car on a saturday.
Then using the car all week.
Then putting the car up the next saturday and biking 6 miles home.
Then used the bike for work all week.
Got stuck in that rut from march 2010 - may 2010 alternating from car to bike ... then one day just grabbed the bike and tried to
do wisbech-sandringham and back.
I did it, 54 miles, nearly killed my ar$e.
Then the next weekend, new gel seat, another long ride of 62 miles.
Ar$e ready to fall off and legs like jelly when i got home! (Gel seats = sweat, friction, heat, rash and pain)!
Then i fitted lightweight wheels and slick tyres, road bike seat, padded trousers and water bottles ... that helped!!
I quickly identified a decent 22 mile circuit around relatively safe fen roads, and did that at least twice a week until december 2010
on this "heavy" but "free running" sitty up bike (24kgs!!!) i then looked to buying a modern road bike.
I think you should do the same: Just make minor changes to sort out pressure points (seat/padded trousers) and maybe switch to
second hand "lightweight" wheels from
ebay with slick tyres ..... then just ride and ride. Work at keeping your legs moving quickly
at any speed to reduce strain on muscles/knees.
You'll know when you're getting fit as you can taste the co2 in your breath when really pushing hard and you'll start seeing things
out the sides of your eyes due to your legs stealing all available glucose from you brain.