Hi to Movingforward, Santa and Duchess!
Movingforward - in answer to your question as to whether cycling slowly is any good- it really depends what your goals are. If you are just wanting to be a bit more active in your life, and to enjoy something that will take you out into the fresh air, then pootling along on your bike is great and will do lots to lift your mood etc. If you would like to extend the length of time that you cycle for, then just build up the distance slowly.
If, on the other hand, you want to cycle for fitness and to lose weight then you will need to do a little more than cycle slowly. Cycling along not using much effort and not exerting yourself is equivalent to going out for a walk. It is great for being more active, but will not increase your fitness or do much for calorie burning.
To achieve these things you need to be exercising in a way that stresses your heart and lungs: so exercise that causes your heart rate to raise significantly and for you to breath hard for a sustained period. If you ain't coming back sweaty, you ain't working hard enough!
There are some on-line calculators such as this one here which will give you a rough idea of how many calories you will burn according to the speed you go at and your weight. It is only a very rough guide, and is not nearly as accurate as working it out from your heart rate. For example, the chart counts cycling at 14-15 mph as 'vigorous' and 16-19 as 'very fast-racing', which is great for someone starting out at cycling, but for someone who is more experienced and fitter, it is rather slow. 18 months ago I would have agreed with those figures, but now for me 19-21 mph is 'vigorous' and 22mph+ is fast. Racing is more like 25-27mph!
You don't have to get technical to make improvements. As a rule of thumb, you will be raising your heart rate to a level where you are working hard enough if you are having to breath hard, but without feeling you are gasping for air. Some simple tips to design your own workout can be to use google maps or similar to measure out a loop, or there-and-back ride, and time yourself over the distance. Keep cycling it a couple of times a week, each time challenging yourself to push quite hard, and then after two week or a month, time yourself again and see if your are doing it any quicker. Or you might want to select a couple of points along your route, and say to yourself, ok, from this signpost to that house I am going to ride as hard as I can. Gradually, as you find it easier, you can make the distance longer. If your route has any hills in it, push yourself to get up that hill as quickly as possible.
One thing that I love about cycling is that I find that I work hard without really thinking about it. I have to confess that on my commute into work, I like to play Silly Commuter Racing, and because I am so focussed on chasing down and claiming a good scalp, I tend not to notice that I have been exerting myself to the max for the past 500m.