Getting into the swing of things

The situation with your legs getting stronger is just time and proper training.   For the time being do not ride any further then you already are, just keep doing the 4 miles to work stuff for another 2 weeks, then, depending on your riding goals, start doing the training program I have given you site to see; http://www.kintera.org/htmlcontent.asp?cid=54755  You don't have to do a century ride but you might want to do maybe a 50 mile ride, so by week 5 you could easily be doing that, or continue to week 8 and do a 100 if you want.  But the only way your going to get stronger is to train, and that training program will do that.  Depending on your schedule probably the best time for you do this training is after work, instead of going 4 miles home you detour the extra miles as given on the schedule.  Do not deviate from that schedule by doing more miles because you feel like you can, because you could end up hurting yourself or overtiring yourself.  don't worry about the easy, pace, brisk stuff yet, just concentrate on building base miles then once you reach your first 100 mile ride then you can go back and work on doing interval training to gain speed.  Also if you notice Thursday is a rest day, if you can take the car or bus in, if you can't then ride a very easy leisurely pace.

I've always been a huge fan of Trek lugged steel bikes, I have a 84 660 myself.  The bike you have is a nice bike and worthy of restoring.

Not knowing your age, condition, what gear you usually ride in, and a bunch of other factors it's hard to say. I do know that personally if I don't roll easily for the first few miles the ride is a little harder. These days 7-10 miles of my usual 35 is a warm-up. And once warmed up properly I can do some pretty spectacular things with my heart, legs, and lungs that would otherwise seem impossible. Furthermore, the right cadence and pacing (this can take a little time to master) can go a long way in determining how long one can last.  

Whatever you do consistency is key. Folks who ride 4x-5x/week vs 3x/week get quite a bit stronger quite a bit quicker.

As far as how long it will take to get into shape really depends on your age, your health, and your own physiology. For me from a layoff of any significant time, it usually takes a month to start feeling a little stronger, and 4 months or so of steady training to get to my best (from a micro view at least... I'd like to think my best years are ahead of me).

PS. If I'm doing 3 regular rides a week of 25-30 miles, I'm more likely to enjoy the 4th ride of 50-60 miles.

Dunno where my "edit" button went but I'll add this - I didn't look at the plan in Froze's link but personally I am under the impression that you will not get much fitter riding 20mins twice a day even if it is 7 days a week. You may get a little stronger but my feeling is one needs at least 45 steady minutes after waming up to see any significant results, preferably at least an hour after the warmup, and at least 4x week (5x is better, but 6x is not that much better). Once in posession of some basic fitness, adding a longer ride of 2-3 hours once a week will likely dramatically improve your fitness.

Nice bike btw, if that "420" had been built in the last decade I'd know what those engineers were up to on their lunch breaks