Everyone is different. I returned to cycling at age 49 after a 35 year hiatus. I led an extremely sedentary lifestyle during that time. Here's how it worked for me.

My first ride, home from the LBS, was only 2/3rds of a mile. I had to stop and rest halfway.

After a month or so I could ride 20 miles with four or five rest stops. Then a three-hour nap afterward, and resting the whole next day.

It was months before my legs didn't hurt 24/7. Then I discovered BikeForums.

I learned the importance of bike fit in ride comfort and prevention of repetitive motion injuries. I also learned I needed to pedal at a faster cadence.

I had a fitting done. Nearly everything about the saddle and handlebar locations was changed. That helped a lot.

I bought a cyclometer with a cadence function and found I was pedaling at under 60 RPM most of the time. While everyone's natural cadence is different, you'll hear that what works best for most folks is between 85 and 95 RPM, or 1.5 times as fast.

This was harder to do than it sounds. When I first tried pedaling faster instead of harder, at 72 RPM I thought my legs would fly out of their hip sockets. It took a whole year before I found the cadence that works best for me is around 95 RPM. By then I could ride 10 miles between rest stops.

So I'm with Jeff and Velo Dog. Start with a fitting--professionally done. Plan on $50 to $150 for this. Ask the fitter too about your cadence. That's what worked for me, though YMMV.

Oh, and one more thing, Greg LeMond was right: It never gets easier, you just get faster.