Cycling gives a good workout for the legs, but not for the arms and shoulders. You will find the riding will be more comfortable and enjoyable if you do some exercises like push-ups. I am not into fitness, but I find this really helps.
Ride as long as you enjoy it, and ocassionally more. Dont try to ride as fast as you can all the time, but save your energy so you can achieve short bursts of power for getting up hills or making a break-away.