Rod,
Congratulations on starting a cycling program and on making positive improvements in your health. Keep up the good work!
The claim that clipless pedals will make you 20% faster is utter nonsense. There is a woman rider who posts in this sub-forum and several others using the name "Machka." She has tremendous long-distance cycling experience; earlier this year, she posted that she had reached the 400,000 km mark in cycling. In another discussion thread about clipless pedals, she stated that in her extensive cycling experience, she is able to ride one kph (= 0.6 mph) faster with clipless pedals. That is nowhere near a 20% increase.
I suggest you take five minutes and read the article at the following link:
http://www.rivbike.com/Articles.asp?ID=255 . It was written by Grant Petersen, the opinionated founder of Rivendell Bicycle Works. In this article he tries to dispel a lot of the hype surrounding clipless pedals and specialized cycling shoes.
I am a 55 year-old rider who has been cycling a year and a half. I use clipless pedals, but it has taken almost this long to warm up to them. I like the feel of them now, and I believe they help promote a smooth pedal stroke over the entire pedal cycle. However I am open to the idea that the effect is not as great as it seems.
I used Shimano SPD-SL pedals/cleats for the first year because that is what my local bicycle shop (LBS) installed on my bike when I bought it. However, I never really liked them--they were too difficult to clip-in after stopping. After a year of frustration, I switched to Speedplay Zeros, and I like them much better. They did require about a one month break-in period, but after that, they were
much easier to clip-in than the Shimano SPD-SL's.
Good luck in your goal of riding a century--I just completed my first one this week. What really helped me improve my cycling was buying a trainer (a Kurt Kinetic Road Machine). This allowed me to "cycle" all winter, and I have continued to use it year round, because it allows me to get in workouts even on days during which I might not ride otherwise. In addition, I was able to easily increase both the duration and intensity of the workouts in a planned, gradual manner, and this is what led to the greatest increases in my cycling fitness. Sure, if I lived in southern California, I might not have the need for a trainer, but for my location and lifestyle, it has really contributed to a steady, noticeable increase in my cycling fitness.