Well, according to Sheldon an rpm of 100 would give me 19.7mph in a 64.9 inch gear. If I'm tooling along on the flat at 20mph, a rare occurence, I'm probably turning a gear around 79 inches and closer to 80rpm.
If you're spinning that high and returning lower speeds then I'd be tempted to say you should be upping the gear you're using straight away. You could be spinning away but not adding that much power through the pedals. This is about you wanting to get stronger and push a bigger gear. Try about 10 inches higher, push it on the flat and get out of the saddle for slopes. I could only do about 20 yards out of the saddle at first. At my fittest I could manage just over a mile out of the saddle, though the hill in question gives you a 100 yard respite in the middle. When I did my esperiment I think I stayed in the same gear for nearly 300 miles. That included a day ride of 115 miles. I cycled from home to the other side of London, got a train to Manningtree, did a social ride, then train back to London. cycled across London and then got train home. I was probably in similar shape then to when I came up to Sheffield. Though without the lack of sleep and the travel up