Probably not the best advice but i'll tell you what worked for me when I first started.
Last early summer I was your typical student, drinking, takeaways, occasional joint etc. A group of us were sat in a pub getting merry when one suggested we do the coast to coast next (this) year. The next day I dusted down my bike and just set off riding. I did around 12 miles in just over an hour and I was knackered, really knackered - aching, sweating, probably bonked etc etc.
I rested for a couple of days and went out again, 20 miles on a flatter route nice and slow. felt good, rested, ate properly etc. next ride was 20 miles but hilly, again nice and slow. A couple of weeks of 2 rides a week and I was off, I got a ride buddy too then the winter just came too quick!
Alongside this I watched what I ate, limited my drinking, knocked the grass on the head. Joined this site for ride tips and motivation. When you see people riding 100m + it spurrs you on to ride further. Now next year me and one friend (an absentee from the pub that night) will be doing the coast to coast and some one day rides/ UK tours this year. We both have the cycling bug and I will warn you, you are constantly looking at routes, new parts and thinig about training plans! And constantly trying to get your mates who don't ride onto a bike too!
Good luck and bon voyage!