OK, let's get this subject dealt with. First of all the thing you are discovering is that bike-riding is not easy at first. Cycling can be the most demanding competitive sport in the World. Having said that, riding along at your own pace on a decent bike on a nice day is actually easier than walking.
Couple of things listed above and I take exception at some-one describing earlier bikes as being made with gas-pipe ! Racing bikes made with 531, 753 etc were very sophisticated for their era and also very lightweight. Frames that I built fifty years ago (Eddie Soens Cycles) with 531 weighed only a few pounds - say 2 kilos, were stiff and responsive. Not just mine but virtually all specialist builders.
Saddle position has been mentioned a couple of times and this really should be determined by a good coach - there are, however, a lot of charlatans out there who, as my late father used to say, "couldn't coach a frog to jump!". So try the following; it is not perfect but a very good compromise:-
1.Set the pedal at the bottom of the stroke and set the saddle height so that your leg is almost straight with your HEEL on the pedal. Make sure that the saddle is horizontal and not running up or down.
2.Set a pedal at 3 o'clock forward and sitting on the saddle with your foot in the normal position ascertain that the angle at your knee is roughly 90 deg. If not, then move the saddle laterally so that this is achieved. This may well then alter the height. so you may need to go thru' item 1 again.
3. Handlebar height and distance. Very difficult this one since some people have abnormal arm lengths but for Mr. Average you will find that you need to ascertain that your arms are slightly bent when riding on the brake levers. If you are reaching too far or if your hands are almost underneath you, then you might have to change the handlebar stem for a longer or shorter one - a very easy thing to do.
Now this isn't perfect since people of differ tremendously in height and shoe size but, as I have said, it is a very good compromise and it may work out perfectly for you.
One thing I often advise people is to get their bike off a former racing cyclist dealer if possible. Even if you have no intention of racing, their expertise can be very valuable. If you buy from a multi-national, be bold and ask the sales-person what qualifications and experience he/she has in giving advice. The question won't be welcomed I would imagine but it's your money and your legs.
Finally, aches, pains and sheer exhaustion at the beginning. Some of this will be caused by your saddle not being correctly set but generally you are beginning to use muscles that you didn't know you had and they are complaining. Someone complained about his backside aching - ah, diddums ! It happens at first but as you get stronger and more used to riding it'll go away.
I remember about sixty years ago (I know, I know !!!) one of the greatest riders of all time, Fausto Coppi, was asked about his immense success and what he did:-
Ride a bike, ride a bike and then ride a bike ! was his advice. Little has altered in between. Just ride at your own pace, choose a sensible gear that matches the terrain and wind and enjoy yourself. Eventually it will all come right for you and things will get so much easier. Cheers. Bill