Clipless pedals (or clips with straps) will allow you to use your entire leg when peddling. Right now you pedal stroke is not much different than climbing stairs, which builds up the fronts of your thighs, and your calves. When you use clipless pedals (or clips and straps) you can use the muscles in the rear of your thighs as you pull up on the pedals. The ideal pedal stroke is to pedal in perfect circles; pushing down, pulling back, pulling up, and pushing forward. The backs of your thighs are more associated with walking and running, and when using clips pedals, add a lot of power to your riding. Using more of your muscles means that your body will burn more carbohydrates and fat.

Be warned that there is a learning curve to clipless pedals, or clips and straps, and that when you use them you are going to fall over at least a few times. It takes awhile for unclipping or unstrapping before a stop to become a habit. But this shouldn't stop you from using them. I ride a bicycle to and from work every day, and it has SPD clipless pedals on it, I wear casual SPD shoes which are also fine for walking around in, and which I don't have to change out of when I am at work. I recommend these type of shoes over racing type shoes, which are not so easy to walk in.

Keep up the good work with the riding. Your weight loss is impressive, but what is more impressive are the results you can't see, such as improvements in your blood chemistry, and a stronger and healthier heart. Keep riding until your body becomes more adjusted to the practice, and used to the endorphins which hard exercise produces. You will find it much easier to get on the bike each morning without forcing yourself to do it. In my case, when I got into serious riding, it was hard to keep myself off the bike.

In regards to whether a road bike will make you faster, of course it will. Part of the improvement is mechanical, as road bike geometry allows for more efficient power transfer and body position. Also road bikes handle much better, they respond more quickly to your movements. The other part is psychological; like a fast car often motivates you to drive faster, a fast bike can do the same.

Frame material is almost completely irrelevant, I can afford any kind of bike or frame material, but I ride a steel bike. I like the lively ride, and the clean look of a classic bicycle. The most important thing when choosing a bike is that it fits you properly. As your fitness improves, your riding position will change, and you will need a bike which can be adjusted to such changes. A good bike shop can get you the proper size frame, and set up the rest to fit you. Eventually, your riding position will become lower, so a bike with an adjustable handlebar stem would be important, drop bars would allow you to shift between a higher and lower riding position.

Get the best bike you can afford, but be sure to budget some money for quality bicycle shorts. There are few things more embarrassing than going out in public in cycling shorts for the first time. But you'll get over it. Good shorts make your ride much more comfortable, and the remove almost all resistance from your legs when pedalling, you will be amazed at the difference in effort when you first try them.

Ride with others if you can, you should be able to find a group ride in your area, don't feel shy about joining, you will find any and all types of people in your typical group ride. Some ride every day, some ride only twice a month. Ride as fast as you feel comfortable with, and if they are going too fast, you can drop out and ride at your own pace. But riding with others is motivating, and fun, and will give you a better workout than riding by yourself.

Keep us posted on your progress,