I started riding in March of '09, commuting to work. My route then was 8 miles, and, at each intersection, I'd hope the light would turn red so I could rest. It wasn't long before I wanted the lights to be all green. Then, I remember how it was hard to breathe enough while also drinking enough (some commutes are at 110+ degrees). That too changed, but it took a bit longer. Eventually, I started looking for ways to increase the commute distance. Then, I got a road bike and started doing weekend club rides. The first few of those were brutal, going from a max distance of 10 miles to 50-60 miles with much faster/fitter riders. But pretty soon, those rides stopped bringing me to my knees.
IMO, there are two ways to approach getting bicycle fit. One way is to gradually increase the distance, in chunks that don't stress your body too much, never really getting wiped out or totally sore. Then there is the way I chose, which is to start doing the distance you want to be able to do, and then keep doing it until it doesn't hurt to do it anymore. That's the "what doesn't kill you, makes you stronger" approach, and it is what worked best for me. Until I started being coached, that is. My coach has this archaic belief that your muscles shouldn't always be sore. That's totally contrary to my motto, which is "If you aren't sore, you're losing the war."
"To each their own" or "It's your thang, do what you wanna do!"
My leap from 10 miles to 60 wouldn't have been nearly as painful if I had a handle on my nutrition as I started doing the longer rides. I wasn't eating enough, and wasn't taking in the needed electrolytes. I bonked bad on my second group ride: both legs cramping, unable to quench my thirst, totally exhausted, laying in the grass at the side of the road, etc. I almost toppled at a stoplight because, when I tried to unclip, my leg said "No, I think I'll just cramp up instead. Then you'll fall over, and I'll get some rest, you moron." And of course, since I was taking so long, the heat built to over 110F. My wife almost divorced me during this period, but after only a few weekends of the longer harder rides, I was able to handle them 'okay'.
So, my recommendation is to (A) get your nutrition and fluids act together now, and then, so long as you have a green light from your physician, (B) push it as hard as you want!
And, really, it's all about the hours.. forget about miles, and just spend as much time in the saddle as possible, at as fast a cadence as you can maintain. That is what will get your fitness up: hours at a good cadence.